Method of selecting between multiple operation modes of providing a broadcast or multicast service Technical field The invention relates to a method of selecting between at least a first and a second operation mode of providing a broadcast or multicast service to user equipments, a method of transmitting an operation expense value to a control entity, a control entity, a base station and a computer program product. Background and related art A challenging task in mobile telecommunication is to provide multimedia services like video services to a wireless user in an efficient way. Efficiency refers here especially to an efficient use of the scarce and expensive radio resource like frequency bands allocated to a network, such that as many users and different services as possible can be served in parallel within the radio frequency band. While existing second generation wireless networks support voice services in an efficient way, third generation wireless systems and packet-based wireless network's goal is to to provide optimal service for data communication and distribution in the most efficient way. One example is mobile TV, which is a service which is provided to subscribers via telecommunication networks. Mobile TV involves providing TV services to mobile phones or in general user equipments (UEs). It combines the services of a mobile phone with television content and represents a logical step both for consumers and operators and content providers. Mobile TV can be provided for example using new generation 3G (WCDMA/HSPA) networks. Such networks provide enough capacities in order to scale-up mobile TV services for the mass market. By using, depending on the service, unlcast or broadcasVmulticast transmission techniques, network capacity and investments can be optimized. Broadcast/multicast techniques are applicable when (multi-media) data has to be provided from one source to a large number of receivers in parallel (e.g. popular TV programs). Unicast techniques are applicable when data has been exchanged between different * communicating entities or data has to be distributed to one or few receivers (e.g. on-demand multi-media content). However, with the provision of media services to customers, the problem has arisen that the limited bandwidth of data transmission has to be distributed effectively among a multitude of wireless users. Multimedia broadcast/multicast service (MBMS) is one of the evolving service concepts within the third generation networks. MBMS is a user service, which is a combination of both a broadcast service and a multicast service. Third generation partnership (3GPP) is working on establishing standards for multimedia broadcasts/multicast services. Regarding such kind of MBMS sen/ice concepts, 3GPP TS 36.300 (third generation partnership project; technical specification group radio access network; evolved universal terrestrial radio access (E-UTRA) and evolved universal terrestrial radio access network (E-UTRAN); overall description; stage 2 (release 8)) suggests two transmission modes for MBMS data, namely point-to-multipoint single-cell (PTM-SC) transmission with full feedback and adaptive modulation and coding applied, but with no coordination with neighboring cells. The second described transmission mode is point-to-multipoint multi-cell transmission (PTM-MC) in the form of an MBSFN (multicast broadcast single frequency network) where participating cells have synchronized MBMS content and thereby considerably reduced interference. However, there Is the technical problem to find a suitable criterion to switch between these two transmission modes. Thereby, information about a density of users interested in an MBMS service is vital for a decision when to switch to PTM-SC or PTM-MC. In PTM-SC mode the MBMS data is sent out Individually in each single cell. This means, that even in the case that cells transmitting the same service are Immediate neighbors, they are not coordinating their transmission and therefore their transmissions might interfere with each other. On the other hand in MBSFN the transmission of MBMS data is synchronized between the different cells such that the signals received by a UE can be soft-combined over multiple cells instead of interfering with each other. This allows that the data can be transmitted more efficiently using less radio resources as for example the frequency in a cell. However, this transmission mode requires that the transmission is performed In a cluster of cells. Therefore, this mode Is only efficient if in a sufficiently large part of the transmitting cells there are interested users for a specific MBMS service. * Furthfermors; it was shown that switching between these two modes depends on the average user density for a given cluster with typical break points between 0.5 and 4 users per cell. This can for example be found in the 3GPP Tdocs R1-071433 'additional results on E-MBMS transmission', Motorola, R1-071049, 'spectral efficiency comparison of possible MBMS transmission schemes: additional results', Ericsson and R1-070984, 'efficiency comparison of MBMS transmission modes', Nokia. Therefore one way to find a suitable criterion to switch between PTM-SC and PTM-MC, is to determine the average number of users per cell for a given MBMS service. Such a determination of the average number of users per cell could for example be performed by counting or polling. Counting thereby identifies how many users take part in a specific MBMS service. The main advantage of this procedure Is the fact, that the average user density can readily be determined. A major disadvantage is the amount of radio resources that needs to be spent. User equipments (UEs) in RRCJDLE may receive an MBMS service without being time aligned, so that they would have to signal their participation via the RACH (random access channel). In case of a large number of participants this may cause congestion (compare 3GPP Tdocs R2-070742, 'counting procedure for LTE MBMS", CATT and R2-062271, 'layer 1 signaling base user detection for LTE MBMS", IP wireless). Polling allows to determine whether there is one or more user equipments present in a cell that is interested in an MBMS service. Thus in contrast to counting, it does not matter how many UEs are interested or participating, but only if they are interested or participating. Generally, polling is regarded to be simpler than counting. In a given cell cluster the average user density, which is determined either directly by counting or implicitly by polling, can also be the criterion for a respective control entity to decide which of the two techniques is more efficient in terms of used ratio resources. Thereby, the control entity is for example the MBMS control entity (MCE). The MCE is responsible for coordination of the base stations taking part in PTM-MC transmission and to select the transmission mode (PTM-SC transmission or PTM-MC transmission) to be applied in the different base stations. Since for the MCE, the quantity of used radio resources is the main focus in the first place, it would be more suitable for a base station to send an estimate of the achievable transmission efficiency e.g. in terms of used resources per standardized data quantity for a given MBMS service in PTM-SC transmission mode and PTM-MC transmission mode instead of polling or counting values to the MCE. This would allow the MCE to have more accurate estimates for the efficiency of service transmission in PTM-SC and PTM-MC mode and to select the most efficient transmission mode for a service. Summary of the invention The present invention provides the method of selecting by a control entity between at least a first and a second operation mode of providing a broadcast or multicast service to user equipments (UEs) in a wireless telecommunication network, the method comprising the steps of transmitting a request signal to a multitude of base stations, the base stations being adapted for providing the service to the user equipments, receiving operating expense values in response to the transmitted request signal from the base stations, the operating expense values describing the amount of radio resources per data volume required for providing the service from the base stations to the user equipments and selecting one of the at least three operation modes of providing the service by analyzing the received operating expense values. An operation mode is in general a transmission technique or a set of transmission techniques that are categorized into said operation mode because of common features. Correspondingly, a change of the operation mode can also refer to a change between two transmission techniques even if they are categorized to be in the same operation mode. Therewith, a base station (in 3GPP LTE: eNodeB) supplies a control entity with information on the estimated radio efficiency, e.g. the amount of radio resources per data volume required for providing the service from the eNodeB to the user equipments, or more exactly the number of used resource blocks per data quantity for a given MBMS service. Since saving radio resources is the primary reason for switching between transmission modes, it is advantageous to directly use a radio resource efficiency value (used spectrum per data quantity) instead of using another value like polling or counting values that only implicitly refer to the same information. Using directly a radio resource efficiency value has further the advantage, that even though radio resource efficiencies might differ between different eNodeSs, either because of differences in the implementation of the eNodeBs or because of different radio environments, the control entity always obtains information about required radio resources which is the key criterion for the control entity to decide which transmission technique to use. In case the radio resource efficiency differs between different eNodeBs, transmitting polling or counting information to a control entity may lead to wrong decisions of the control entity which transmission technique to use, because the control entity has no information about the different radio resource efficiencies at the different eNodeBs. Therewith, by directly using radio resource efficiency values transmitted from the eNodeBs to the control entities has the advantage of obtaining a more precise basis for the decision of the control entity which transmission technique to use. It has to be mentioned, that transmitting the request signal to the multitude of base stations (eNodeBs) might be only necessary when initially providing a given service to user equipments via the eNodeBs. For example, if the eNodeBs support automatic and periodic transmission of used radio resources to the control entity, the control entity does have to send said request signal only once. After that, the control entity periodically receives radio resource efficiency values from the base stations. In accordance with an embodiment of the invention, the service is a multimedia broadcast/multicast service (MBMS). This allows a highly efficient distribution of multimedia data streams to user equipments in the digital wireless communication network. MBMS offers a capacity for data transmission which allows for multicast distribution instead of only a point-to-point link to user equipments in the digital wireless communication network. This leads to a more efficient system usage enabling streaming and download services for a multitude of user equipments served by a respective eNodeB. The operation modes or transmission techniques of providing a broadcast or multicast service to user equipments may be categorized by taking into account at least the feature "synchronicity over multiple cells" and the feature "existence of a feedback mechanism". The broadcast and multicast service by nature in MBMS transmits data point-to-multipoint, but does not preclude point-to-point transmission for low user density cases. In accordance with an embodiment of the invention, the first operation mode is characterized in that the providing of the service from the base stations to the user equipments is scheduled autonomously by the base stations. The second operation mode is characterized in that the providing of the service from the base stations to the user equiprnents is scheduled by the control entity. In other words, in the first operation mode the base station is allowed to set and adjust for example transmission frequencies, timeslots and the modulation and coding scheme (MCS), whereas in the second operation mode transmission frequencies, timeslots and MCS are specified by the control entity. For example, the first operation mode is a point-to-multipoint single-cell transmission mode (PTM-SC) and the second mode is a point-to-multipoint multi-cell transmission mode (PTM-MC), It has to be noted that in case the control entity configures the base stations to use the first operation mode, even though the base stations decide on the radio resources and configuration they apply for transmitting the service in an autonomous way, this case does not preclude that the control entity provides base stations with information, e.g. frequencies to be used only with restricted power, for the control of interference between neighbouring base stations. Conversely, in addition to the expense value reports the base stations can also add supplementary information contained in these reports, lil