Add support for offset coordinates

[abeelen]

I am reopening astrofrog/wcsaxes#8
I tried the SkyOffsetFrame solution, but the result is not satisfactory, considering this example :

import numpy as np
import matplotlib.pyplot as plt
from astropy.wcs import WCS
from astropy.coordinates import SkyCoord

header = {'CRVAL1': 15., 'CRVAL2': 20.,
          'CUNIT1': 'deg', 'CUNIT2': 'deg',
          'CRPIX1': 16, 'CRPIX2': 16,
          'CTYPE1': 'RA---TAN', 'CTYPE2': 'DEC--TAN' }
wcs = WCS(header)
coord = SkyCoord(header['CRVAL1'], header['CRVAL2'], unit="deg")

ax = plt.subplot(projection=wcs)
ax.imshow(np.zeros((32, 32)))
overlay = ax.get_coords_overlay(coord.skyoffset_frame())

the overlay axes looks good for the latitude but is an angle module 360° for the longitude.

[astrofrog]

You can fix this by using:

from astropy import units as u
overlay[0].set_coord_type('longitude', 180)

However it would be nice to have this be automatic for offset frames.

[keflavich]

I have a similar use case; I had to do the following to get a reasonable display:

ra = ax3.coords['ra']
ra.set_auto_axislabel(False)
dec = ax3.coords['dec']
ra.set_coord_type('longitude', 180)
overlay = ax3.get_coords_overlay(offset_loc.skyoffset_frame())
ra.set_ticklabel_visible(False)
dec.set_ticklabel_visible(False)
ra.set_ticks_visible(False)
dec.set_ticks_visible(False)

lon = overlay['lon']
lon.set_coord_type('longitude', 180)
lon.set_format_unit(u.arcsec)
lon.set_ticklabel(rotation=45, pad=40)
lon.set_ticks_position('b')
lon.set_ticklabel_position('b')
lon.set_axislabel_position('b')
lat = overlay['lat']
lat.set_format_unit(u.arcsec)
lat.set_ticklabel()
lat.set_ticks_position('l')
lat.set_ticklabel_position('l')
lat.set_axislabel_position('l')

I'd like to add this to the documentation, but I think we should also make this much easier (maybe it already is?). It feels like I had to do a huge amount of manual relocation here.