METHODS FOR MAKING HOLOGRAPHIC DATA STORAGE ARTICLES BACKGROUND The present disclosure relates to methods for making and using holographic data storage articles. Further, the disclosure relates to holographic data storage articles. Holographic storage is the storage of data in the form of holograms, which are images of three dimensional interference patterns created by the intersection of two beams of light, in a photosensitive medium. The superposition of a signal beam, which contains digitally encoded data, and a reference beam forms an interference pattern within the volume of the medium resulting in a chemical reaction that changes or modulates the refractive index of the medium. This modulation serves to record as the hologram both the intensity and phase information from the signal. The hologram can later be retrieved by exposing the storage medium to the reference beam alone, which interacts with the stored holographic data to generate a reconstructed signal beam proportional to the initial signal beam used to store the holographic image. Thus, in holographic data storage, data is stored throughout the volume of the medium via three dimensional interference patterns. Each hologram may contain anywhere from one to 1x10^ or more bits of data. One distinct advantage of holographic storage over surface-based storage formats, including CDs or DVDs, is that a large number of holograms may be stored in an overlapping manner in the same volume of the photosensitive medium using a multiplexing technique, such as by varying the signal and/or reference beam angle, wavelength, or medium position. However, a major impediment towards the realization of holographic storage as a viable technique has been the development of a reliable and economically feasible storage medium. Early holographic storage media employed inorganic photo-refractive crystals, such as doped or un-doped lithium niobate (LiNbO3), in which incident light creates refractive index changes. These index changes are due to the photo-induced creation and subsequent trapping of electrons leading to an induced internal electric field that ultimately modifies the refractive index through a linear electro -optic effect. However, LiNbO3 is expensive, exhibits relatively poor efficiency, fades over time, and requires thick crystals to observe any significant index changes. More recent work has led to the development of polymers that can sustain larger refractive index changes owing to optically induced polymerization processes. These materials, which are referred to as photopolymers, have significantly improved optical sensitivity and efficiency relative to LiNbO3 and its variants. In prior art processes, "single-chemistry" systems have been employed, wherein the media comprise a homogeneous mixture of at least one photo-active polymerizable liquid monomer or oligomer, an initiator, an inert polymeric filler, and optionally a sensitizer. Since it initially has a large fraction of the mixture in monomeric or oligomeric form, the medium may have a gel-like consistency that necessitates an ultraviolet (UV) curing step to provide form and stability. Unfortunately, the UV curing step may consume a large portion of the photo-active monomer or oligomer, leaving significantly less photo-active monomer or oligomer available for data storage. Furthermore, even under highly controlled curing conditions, the UV curing step may often result in variable degrees of polymerization and, consequently, poor uniformity among media samples. Dye-doped data storage materials based on polymeric materials have been developed. The sensitivity of a dye-doped data storage material is dependent upon the concentration of the dye, the dye's absorption cross-section at the recording wavelength, the quantum efficiency of the photochemical transition, and the index change of the dye molecule for a unit dye density. However, as the product of dye concentration and the absorption cross-section increases, the storage medium (for example, an optical data storage disc) becomes opaque, which complicates both recording and readout. Therefore, there is a need for holographic data storage' methods whereby high volumetric data storage capacities can be achieved using photochemically active dyes that are efficient and sensitive to electromagnetic energy, such as light without interference from the main absorption peak of the dye. SUMMARY Disclosed herein are methods for producing and using holographic data storage media, which are valuable for reliably storing large amount of data. In one aspect, the present invention is a method of making a holographic data storage medium. The method comprises: (a) providing an optically transparent substrate comprising at least one photochemically active dye; and (b) irradiating the optically transparent substrate at at least one wavelength at which the optically transparent substrate has an absorbance in a range from about 0.1 to 1, to produce a modified optically transparent substrate comprising at least one optically readable datum and at least one photo-product of the photochemically active dye. The at least one wavelength is in a range from about 300 nanometers to about 800 nanometers. The optically transparent substrate is at least 100 micrometers thick, and comprises the photochemically active dye in an amount corresponding to from about 0.1 to about 10 weight percent based on a total weight of the optically transparent substrate. In another aspect of the present invention, an optical writing and reading method is provided. The method comprises irradiating a holographic data storage medium with a signal beam possessing data (or at least one datum) and a reference beam simultaneously to partly convert the photochemically active dye into at least one photo-product and store the data in the signal beam as a hologram in the holographic data storage medium. The holographic storage medium comprises an optically transparent substrate and at least one photochemically active dye. The optically transparent substrate has a thickness of at least 100 micrometers, and comprises the photochemically active dye in an amount corresponding to from about 0.1 to about 10 weight percent based on a total weight of the optically transparent substrate, and a UV-visible absorbance in a range from about 0.1 to 1 at at least one wavelength in a range from about 300 nanometers to about 800 nanometers. Then the holographic storage medium is irradiated with a read beam and the data contained by diffracted light from the hologram is read. In an embodiment, conversion of the photochemically active dye to at least one photo-product occurs such that the data storage medium comprises the dye as well as the photo-product to provide the refractive index contrast needed to produce the hologram. In yet another aspect, the present invention is a method for using a holographic data storage article. The method comprises irradiating a holographic data storage medium in the holographic data storage article with electromagnetic energy having a first wavelength. The holographic data storage medium comprises an optically transparent substrate that is at least 100 micrometers thick, and comprises at least one photochemically active dye in an amount corresponding to from about 0.1 to about 10 weight percent based on a total weight of the optically transparent substrate. The irradiation is done at at least one wavelength in a range from about 300 nanometers to about 800 nanometers at which the optically transparent substrate has a UV-visible absorbance in a range from about 0.1 to 1. A modified optically transparent substrate comprising at least one photo-product of the at least one photochemically active dye, and at least one optically readable datum stored as a hologram is formed. Then the modified optically transparent substrate is irradiated with electromagnetic energy having a second wavelength to read the hologram. In still yet another aspect, the present invention is a method for manufacturing a holographic data storage medium. The method comprises forming a film of an optically transparent substrate comprising at least one optically transparent plastic material and at least one photochemically active dye having a UV-visible absorbance in a range between about 0.1 and about 1 at a wavelength in a range between about 300 nanometers and about 800 nanometers, said film having a thickness of at least 100 micrometers;, wherein the optically transparent substrate comprises from about 0.1 to about 10 weight percent of the optically transparent substrate. In another aspect, the present invention is a holographic data storage medium. The holographic data storage medium comprises an optically transparent substrate comprising at least one optically transparent plastic material, at least one photochemically active dye, and at least one photo-product thereof. The at least one photo-product is patterned within the optically transparent substrate to provide at least one optically readable datum comprised within the holographic storage medium. The optically transparent substrate is at least 100 micrometers thick and comprises the photochemically active dye in an amount corresponding to from about 0.1 to about 10 weight percent based on a total weight of the optically transparent substrate. The optically transparent substrate has a UV-visible absorbance in a range from about 0.1 to 1 at at least one wavelength in a range from about 300 nanometers to about 800 nanometers. These and other features, aspects, and advantages of the present invention may be more understood more readily by reference to the following detailed description. DETAILED DESCRIPTION Some aspects of the present invention and general scientific principles used herein can be more clearly understood by referring to U.S. Patent Application 2005/0136333 (Serial Number 10,742,461), which was published on June 23, 2005; and co-pending Application having Serial Number 10/954,779, filed on September 30, 2004; both which are incorporated herein in their entirety. As defined herein, the term M/# denotes the capacity of a data storage medium, and can be measured as a function of the total number of multiplexed holograms that can be recorded at a volume element of the data storage medium at a given diffraction efficiency. M/# depends upon various parameters, such as the change in refractive index (An), the thickness of the medium, and the dye concentration. These terms are described further in this disclosure. The M/# is defined as shown in equation (1): (Equation Removed) where ŊI is diffraction efficiency of the ith hologram, and N is the number of recorded holograms. The experimental setup for M/# measurement for a test sample at a chosen wavelength, for example, at 532 nanometers or 405 nanometers involves positioning the testing sample on a rotary stage that is controlled by a computer. The rotary stage has a high angular resolution, for example, about 0.0001 degree. An M/# measurement involves two steps: recording and readout. At recording, multiple planewave holograms are recorded at the same location on the same sample. A plane wave hologram is a recorded interference pattern produced by a signal beam and a reference beam. The signal and reference beams are coherent to each other. They are both planewaves that have the same power and beam size, incident at the same location on the sample, and polarized in the same direction. Multiple planewave holograms are recorded by rotating the sample. Angular spacing between two adjacent holograms is about 0.2 degree. This spacing is chosen so that their impact to the previously recorded holograms, when multiplexing additional holograms, is minimal and at the same time, the usage of the total capacity of the media is efficient. Recording time for each hologram is generally the same in M/# measurements. At readout, the signal beam is blocked. The diffracted signal is measured using the reference beam and an amplified photo-detector. Diffracted power is measured by rotating the sample across the recording angle range with a step size of about 0.004 degree. The power of the reference beam used for readout is typically about 2-3 orders of magnitude smaller than that used at recording. This is to minimize hologram erasure during readout while maintaining a measurable diffracted signal. From the diffracted signal, the multiplexed holograms can be identified from the diffraction peaks at the hologram recording angles. The diffraction efficiency of the ith hologram, T|J,, is then calculated by using equation (2): (Equation Removed) where Pi, diffracted is the diffracted power of the ith hologram. M/# is then calculated using the diffraction efficiencies of the holograms and equation (1). Thus, a holographic plane wave characterization system may be used to test the characteristics of the data storage material, especially multiplexed holograms. Further, the characteristics of the data storage material can also be determined by measuring the diffraction efficiency. As defined herein, the term "volume element" means a three dimensional portion of the total volume of an optically transparent substrate or a modified optically transparent substrate. As defined herein, the term "optically readable datum" can be understood as being made up of one or more volume elements of a first or a modified optically transparent substrate containing a "hologram" of the data to be stored. The refractive index within an individual volume element may be constant throughout the volume element, as in the case of a volume element that has not been exposed to electromagnetic radiation, or in the case of a volume element in which the photochemically active dye has been reacted to the same degree throughout the volume element. It is believed that most volume elements that have been exposed to electromagnetic radiation during the holographic data writing process will contain a complex holographic pattern and as such the refractive index within the volume element will vary across the volume element. In instances in which the refractive index within the volume element varies across the volume element, it is convenient to regard the volume element as having an "average refractive index" which may be compared to the refractive index of the corresponding volume element prior to irradiation. Thus, in one embodiment an optically readable datum comprises at least one volume element having a refractive index that is different from a (the) corresponding volume element of the optically transparent substrate prior to irradiation. Data storage is achieved by locally changing the refractive index of the data storage medium in a graded fashion (continuous sinusoidal variations), rather than discrete steps, and then using the induced changes as diffractive optical elements. The capacity to store data as holograms (M/#) is also directly proportional to the ratio of the change in refractive index per unit dye density (An/NO) at the wavelength used for reading the data to the absorption cross section (a) at a given wavelength used for writing the data as a hologram. The refractive index change per unit dye density is given by the ratio of the difference in refractive index of the volume element before irradiation minus the refractive index of the same volume element after irradiation to the density of the dye molecules. The refractive index change per unit dye density has a unit of (centimeter)3. Thus in an embodiment, the optically readable datum comprises at least one volume element wherein the ratio of the change in the refractive index per unit dye density of the at least one volume element to an absorption cross section of the at least one photochemically active dye is at least about 10"5 expressed in units of centimeter. Sensitivity (S) is a measure of the diffraction efficiency of a hologram recorded using a certain amount of light fluence (F). The light fluence (F) is given by the product of light intensity (I) and recording time (t). Mathematically, sensitivity is given by equation (3), (Equation Removed) wherein I is the intensity of the recording beam, "t" is the recording time, L is the thickness of the recording (or data storage) medium (example, disc), and rj is the diffraction efficiency. Diffraction efficiency is given by equation (4), (Equation Removed) wherein X is the wavelength of light in the recording medium, 8 is the recording angle in the media, and An is the refractive index contrast of the grating, which is produced by the recording process, wherein the dye molecule undergoes a photochemical conversion. The absorption cross section is a measurement of an atom or molecule's ability to absorb light at a specified wavelength, and is measured in square cm/molecule. It is generally denoted by