JSA: set product header for pasted healpix preview

Now sets "product" as well as "productID".
general: add tilenum to PNG headers
JSA: add jsa_pub_asn_id methods

Added this method to the SCUBA-2 and ACSIS frame classes, overriding
a dummy method in the JSAFile base class.  For SCUBA-2 this is just
850um or 450um.  For HARP it follows the rules on ticket #376.
For other instruments it should return undef for now.  Note that the
tile number is not included at this stage.
JSA: tag reduced JSA tile previews as healpix

_TAG_AS_REDUCED_PRODUCT_ now puts "healpix" instead of "reduced"
in the product and product ID when in JSA tiles mode.
JSA: update _COADD_JSA_TILES_ primitive

The primitive now puts all the files for each tile into a
temporary frame before starting to process them.  It then
gets the JSA public association identifier using the new
jsa_pub_asn_id method.  The actual work of combining
the tiles has been broken out into two primitives,
_COADD_JSA_TILES_SCUBA2_ and _COADD_JSA_TILES_ACSIS_
as per my task in Trac ticket #361.  Only the first
of these has been implemented.
JSA: FCF calibration of SCUBA-2 tiles in co-add

After talking to JTD and SFG about calibration of SCUBA-2
public tiles today, moved the FCF calibration step from
REDUCE_SCAN_JSA_PUBLIC into _COADD_JSA_TILES_SCUBA2_.
The intermediate ("obs" level) tiles will be calibrated
in pW for the time being.