Yet another wcs fix. The previous version was internally consistent, but shifted by 1 pixel compared to the FITS standard.
Fixed bug in pmat_thumbs
Merge branch 'master' of uio:git/repos/enlib
Added ref argument to the wcs builders, which specifies a point we wish to maintain pixel compatibility with. So our pixels will be the same as if crval had been at that location with crpix = 0.