polarimetry: Use readhdr not mergehdr to propagate the headers.
JSA: Expand the description and role of SUFFIX argument.
JSA: Add COMMENT argument.

It seems odd to not add a comment, even if it is only the generic
"productID" to indicate what the keyword is for.  It can also permit a
longer description than the terse productID value itself.
polarimetry: Revise tagging and naming of products.

 - Removed the spectroscopy (sp) and imaging (sp) prefixes
 - Used clearer description productIDs, e.g. "percent" for "P"
 - Introduced comments to the PRODUCT headers.
 - Removed _SET_FILE_FITS_ITEM_ calls as the comment and clearer
   productIDs means we only need the invocation of
   _SET_FILE_FITS_ITEM_ within _TAG_AS_REPRESENTATIVE_.
imaging: Revert to just displaying the vector overlay.

Dealing with the FITS table as a product needs more work, e.g.
propagating the required metadata, and not just for UKIRT.
polarimetry: Add comments about the FITS table as a product.