SEP for the re-implementation of `sunpy.wcs` as `sunpy.coordinates`

Putting in the PR so that people can form their opinions on what I've written so far.
Fire away.
Is the 1AU where we base the point of view? I mean, if we put an observer in Solar Orbiter (or Mercury, or somewhere else) the arcsec in the helioprojective coordinates are different... which is not what we are doing here... right? that would be wcs task? here is about the coordinates and not about the point of view... or am I wrong?

I think now I'm more confused....