Enhancement/implement lonlat output

• Implemented 3 new config parameters: 'target_projection_type', 'target_epsg' and 'target_coord_grid'. This allows the user to choose between UTM and geographic L2A projection, to specify a custom L2A projection by providing an EPSG code or to specify a certain L2A coordinate grid.
• Added 'grid_res' and 'epsg' attributes to EnMAP_Metadata_L2A_MapGeo object.
• The L2A projection metadata is now correctly written to the XML file.

Closes issue #55 (closed).

@segl @nbohn