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.
Perfect! now it makes completely sense!!! and I realised that I got that in my mind somewhere but it had slip away somehow... Good, then it's perfect.  I wonder... would not it be better for this SEP to have some diagram?