Consider having a wcscheck utility

In the same way as there are ``fitscheck`` and ``volint`` utilities, it would be handy to have a ``wcscheck`` or ``wcslint`` utility that could easily check whether the WCS in a FITS file or header file is valid, and print out warnings for known non-standard uses.

@mdboom - if you think this would not be useful, or impossible, feel free to close the issue.