Accept `CompImageHDU` objects

`aplpy` currently raises an exception if a `FITSFigure` is instantiated using an instance of `astropy.io.fits.CompImageHDU`. Is there a reason behind this, or was this type of HDU simply overlooked?

This pull requests allows `CompImageHDU` to be accepted. These HDUs work perfectly fine with `aplpy` on the images I have tested.