Account for changes to PC and CD matrix when unwrapping to a FITS WCS that has been rebinned.

[wtbarnes]

This addresses part of the refactor of sunpy.map.GenericMap on top of ndcube.NDCube. See Map-NDCube Refactor (view) for more details.

This PR adds two corrections to the way the CD matrix is constructed when unwrapping a WCS object to a FITS WCS in the case where a rebinning operation is being performed:

1. This adds a correction factor for rebinning to the CD matrix in the resulting FITS WCS upon unwrapping as is already done in Map, https://github.com/sunpy/sunpy/blob/1a2f071811327e03c0c92914f30bbbdadebd02fb/sunpy/map/mapbase.py#L2289-L2292
2. This adds the correction to the non-diagonal PC matrix elements consistent with what is done in sunpy.map.GenericMap.superpixel, i.e. https://github.com/sunpy/sunpy/blob/1a2f071811327e03c0c92914f30bbbdadebd02fb/sunpy/map/mapbase.py#L2293-L2295.

TODOs

• Explanatory comments
• Unit test
• Integration test