shbasis: Changes to permit conversion of erroneous data

Conversion of SH image data is now handled using a single option, "-convert", which is type_choice().
The previous options are there (old, new, native), as well as two new ones (force_oldtonew, force_newtoold): these allow conversion of data where shbasis cannot unambiguously determine the SH basis of the underlying data.
Closes #100.