[AstroPy] External packages in astropy

[Thomas Robitaille]

> The SIP convention extends the CTYPE keywords so they have a "SIP"
> suffix, for example "RA---TAN-SIP".  wcslib by default rejects this as
> being an invalid CTYPE.  astropy.wcs/pywcs includes SIP support, so we
> don't want to raise an exception.
>
> This could be handled outside of wcslib by changing these values before
> passing them in, which would give us compatibility with vanilla wcslib
> -- that might be a reasonable solution.

I agree that if we could make it so that we ensure that we can use the
vanilla WCS and dealing with and stripping out the SIP stuff before
passing the header to wcslib, this would be a good solution that would
make most people happy. Then wcslib could be moved to cextern, and
disabled in package managers than include wcslib (with the caveat that
only recent versions of wcslib would be supported due to recent bug
fixes). Maybe Astropy could check the version of wcslib being used and
raise an error if it is too old?

Cheers,
Tom