Make WCS comparison more permissive

I ran into an issue today where WCS comparison between mask and data was too restrictive.  The problems were:

 * CDELT was stored with float32 precision in one header and float64 precision in the other
 * One header included an equinox and the other did not

This PR solves these by changing missing header cards to "just a warning" and using `np.testing.assert_almost_equal` for float comparison.