From position in the projection plane to angular direction (Mollweide).
- Input:
- x : if y is None, x[0], x[1] define the position in Mollweide plane.
- y : if defined, x,y define the position in projection plane.
- lonlat: if True, angle are assumed in degree, and longitude, latitude
- flipconv is either ‘astro’ or ‘geo’. None will be default.
- Return:
- theta, phi : angular direction.