JSA: correct dummy version of jsa_pub_asn_id

The implementation in the ORAC::JSAFile base class was supposed
to take the place of an abstract / pure virtual method in other
languages, so it was meant to raise an error rather than returning
a dummy string if called.
JSA: tag JSA tile co-adds in group context

Co-added JSA tiles are now tagged as the reduced product (actually
"healpix") in group context so that the pipeline sets the ASN_TYPE
header to "night" (for jsawrapdr to correct later) and adds the
ASN_ID header.  This required the creation of a temporary group
and improved intermediate tracking.  The tagged product is now
added to the list of intermediates so the keep pattern needed to
be corrected from "reduced" to "healpix" to prevent the products
from being deleted.
JSA: add g prefix to JSA tile co-add filenames
JSA: preserve frame/group context when untrimming

_UNTRIM_JSA_TILES_ needed a GROUP option so that it can work
on a group in order for the preview images for JSA tile co-adds
to have the correct jsa:asn_type and jsa:asn_id headers.