LONGITUDE plane: allow user to explicitly choose East or West longitudes.

[ossama-othman]

The LONGITUDE map plane currently ends up containing West longitudes if the body rotation is prograde, and East longitudes if the rotation is retrograde, both in the [0, 360] degree range.

Add LONGITUDE map plane options that allow the user to choose the type of longitudes embedded the plane, i.e. EAST or WEST, as well as the range [0, 360] or [-180, 180].