Add map_variables argument to get_pvgis_tmy

[AdamRJensen]

With the addition of the get_pvgis_hourly function (#1186) a variable map dictionary for PVGIS has been added to the PVGIS iotools module.

This issue is raised to add a map_variables boolean input to the read_pvgis_tmy and the get_pvgis_tmy functions to follow the standard for the iotools functions.

While it is a minor change, it is a breaking change as the default for map_variables should be True.

Note: the pvgis_hourly and pvgis_tmy functions should use the same variable map dictionary, hence this feature should first be implemented once #1186 has been merged.