- Replace packed GSL matrix with more sensible/extensible struct for
  storing coordinate transform data
- Use FITS table instead of array to write coordinate transform data,
  which will BREAK existing FITS files!
- Test FITS file writing in SuperskyMetricsTest
- FIXME: replace 'SMAX' with either 'nspins', 'nsky_offsets - 1',
  or something else...
Original: 77769cf401b07360e4091b8bbdd2513c241bf211