Add low-wind turbines

[fneum]

Tobias showed in his seminar presentation in the Energy Informatics seminar that the choice of wind turbine type can have a significant impact on the optimal investment. Currently, PyPSA-Eur only considers one turbine type for onshore wind generation. An extension would be -- as a tradeoff between increased complexity through additional decision variables and offering choice of turbine type -- to include in addition to the default Vestas V112 a typical low-wind turbine as investment option.