Add wtg parser
Add WindTurbines.from_WAsP_wtg(cls, wtg_file, power_unit='W') to allow using .wtg file/files for initializing one/multiple types of turbines.
Note that .wtg file is used in WAsP for storing turbine information using xml format, thus, this requires using xml.etree.ElementTree.
The main function has been extended to demonstrate how to do this initialization with two .wtg files for two turbines, Vestas V80 and NEG-Micon 2750.