Added Full Solar Collector Model
- DNI numerical problems resolved (no more division by DNI)
- the collector has some power input even during defocusing (through S_eff)
- explicit Defocusing singal
- added a dynamic wall
the efficiency is still correct, since the internal and external walls have the same temperature
doubts: is the energy balance ok? some doubts since the p_conv_fluid is calculated in the absorber model and not at the internal wall

Problems:
- at the moment is not possible to set the initialization for the internal wall temperature
- automatic choiche of colletor is not yet implemented
Can it work with Enumeration? Enumeration uses variables, which can not be passed to the sub-model via initialization...