Added the Helioprojective coordinate system.
1. The default representation is Cartesian, but the units are in degrees!
2. zeta is the z-coordinate in this system and is defined as zeta = D0 - d.
3. In radial representation, although we use Trho, the system must be defined as (Drho, psi) where Trho = Drho + 90 degrees. Need some opinion on how to define this system properly.