Made some changes as pointed out by Cadair.
1. Removed d from frame attributes of heliocentric frame. It is calculated during transformation from HCC to HPC as the distance between the points (x,y,z) and (0,0,D0) instead. D0 remains a frame attribute.
2. Both d and D0 default to 1AU. D0 does *not* default to 1RSUN as thought earlier.
3. d and D0 are user specifiable in the helioprojective frame. This is because d cannot be calculated in a projective frame.
4. In HPC to HCC, use d and D0 of HPC coord and not HCC frame to transform.
5. Removed the 'specified in kilometres' bit from class docstrings.