From unit vector direction to position in the projection plane (Cartesian).
- Input:
- vx: if vy and vz are None, vx[0],vx[1],vx[2] defines the unit vector.
- vy,vz: if defined, vx,vy,vz define the unit vector
- 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.