Observatory location: use ITRS cartesian instead of geodetic

[kosack]

I just noticed the FITS standard says the ITRS cartesian coordinates OBSGEO-X/Y/Z are strongly preferred to geoodetic ones OBSGEO-L/B/H, where ns accuracy is needed. So I think we should switch the required metadata to be the ITRS version. It's less human-friendly to read, but is easy to compute with astropy.