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.
ok, so the 1*au is a bit silly, because it is specifying a point deep in the Sun.

If I make the example 

```
sc = SkyCoord(1100*u.arcsec, 0*u.arcsec, 1*u.au, frame='helioprojective)
```

does it make a little more sense?