JSA: Add DISPLAY argument.

Sometimes we want to postpone display such as for polarimetry.
lib: Added JSA to path for UKIRT Cassegrain instruments.

This might change if we decide to have a UKIRTSA directory.  For now
reuse the JSA code.
imaging: First go at adding preview image and PRODUCT for UKIRT Cass Archive.

Same change as for the generic _MAKE_MOSAIC_.
imaging: Add preview images for polarimetry.

POLPACK loses the FITS airlock, so this has to be copied first (from
the E-beam at 0 degrees) so that we can satisfy CADC requirements and
insert the PRODUCT keyword.  Some of the specific headers from
the copied airlock should probably be removed.
spectroscopy: Add preview images for polarimetry.

See remarks for 3fddddfe33561, which also apply here.
spectroscopy: Add preview images.
heterodyne: Recalculate the number of rejected spectra.

This was messed up when I added the dilation.