smurf: Factor out some CFITSIO code

Move CFITSIO header reading code and CFITSIO error to Starlink
error macro to a libsmfftsio/ library inside SMURF. The code
will be used by other routines that need FITS access. Eventually
this code could migrate to CVG.
smurf: Improve an error message
smurf: Add NANTEN2 SMART heterodyne instrument importer

Similar to the Supercam importer in that it uses the CLASS FITS binary
table format. The NANTEN2 exporter writes all spectra to a single
file with one row per spectrum.

Some issues remaining with data model mapping but the basic system
is functional.