Fix/wcs projection 606

[cvanelteren]

Closes #606
Issue #606 is that UltraPlot does not correctly handle Astropy WCS projection objects passed as projection=mosaic.wcs.

In practice, this shows up when code like ax.get_transform("icrs") fails with:

TypeError: Artist.get_transform() takes 1 positional argument but 2 were given

The underlying problem is:

1. UltraPlot was not preserving/wrapping non-string projection objects like WCS during axes creation.
2. The external-axes container then exposed the wrong get_transform() method, so WCS-specific transform lookups were
   masked by the base Matplotlib artist API.

[cvanelteren]

Integration requires more effort see #606 -- don't have time to work on this atm.