Add utilities for manipulating WCS objects

These are some utilities I have recently needed, in particular for removing the (degenerate) Stokes axis from CASA-generated FITS cubes.

Feedback would be very helpful - am I preserving all the appropriate metadata?  Is this an acceptable place to put utilities?

There are other utility functions I'd like to include here, e.g. `get_platescale` (or `get_pixscale`) from https://github.com/aplpy/aplpy/blob/master/aplpy/wcs_util.py#L83.