From angular direction to position in the projection plane (Cartesian).
- Input:
- theta: if phi is None, theta[0] contains theta, theta[1] contains phi
- phi : if phi is not None, theta,phi are direction
- lonlat: if True, angle are assumed in degree, and longitude, latitude
- flipconv is either ‘astro’ or ‘geo’. None will be default.
- Return:
- x, y: position in Cartesian plane.