Workaround for reading large FITS files on MacOS X

@embray - since I really needed this to work without upgrading OS X I implemented a workaround which doesn't actually seem too hacky. Is there a reason we can't include this? (Fixes #3078)
Maybe don't do this check at module level, since it's a bit slow (it has to read and parse an XML file).  And check `sys.platform == 'Darwin'` first; might as well.  Maybe the check should be moved to inside `_array_from_file` is it isn't done at module level (though the result could be cached in a module-level variable).