spectroscopy: Add PRODUCT keyword for standard.

Use the ad hoc approach for now.  Will replace with product() method
calls once that accepts a header comment.
spectroscopy: Add PRODUCT keyword for arc.
heterodyne: Refix UNMAKECUBE to run with usedetpos=no.

This prevents a one-pixel offset in some cases, which can lead to an
all-bad time-series cube being generated.
heterodyne: Use ORAC temporary files for file lists.

They can be retained for debugging unlike the explicit file names,
which were explicitly deleted in the code.