solar thermal collector

Related to #293 (closed).

1. I identified to PartialHeatGeneration models. I extended the SolarThermal from AixLib.Fluid.BoilerCHP.BaseClasses.PartialHeatGenerator and deleted AixLib.Fluid.HeatExchangers.BaseClasses.PartialHeatGen since the other one is already correctly paramtrized according to the IPBSA conventions.
2. Efficiency is now calulated based on the mean temperature of the collector.
3. Also eta_zero is used as upper limit for eta.
4. Minor improvements, e.g. regarding units.

I did not change parameter and variable names to meet the naming conventions. Otherwise conversion scripts would be necessary.

Furthermore, the PartialHeatGenerator model is not only used in the BoilerCHP subpackge. Maybe it should be moved some day.

Since @KFinkbeiner still wants to compare our solar collector to other existing ones, issue #293 (closed) can stay open.

But this branch can be deleted after a successful merge.