Overhauled the conversion framework for Helioprojective(Cartesian) and Heliocentric(Cartesian).
1. Changed RSUN to RSUN_METERS and added a DSUN_METERS.
2. Minor changes to storing the 'rad' attribute in Heliographic frames in kilometers.
3. Changed HPC to HCC and HCC to HPC entirely, with the help of functions in sunpy.wcs. This was the intention all along.