[Review]: Rotation improvements

An AIA submap is working but my HMI submap test is not working. More details to follow.

Assistance appreciated.
Is that a philosophical statement?  Because HMI data has NaNs, so we have to deal with it somehow.  scikit-image can deal with NaNs while SciPy cannot, so we need to explain the difference.

Restoring the NaNs is non-trivial because presumably we'd have to create a mask, rotate it, and then reapply it afterwards.  You can create an issue after this PR is merged.