spectroscopy: Rename extract collapsed spectrum portion of arc.

We wanted the _arc suffix for the Science Archive rather than
use a different pattern match of arc_<obsid>...  So rename the
extracted and collapsed spectra _sparc for spectrum arc.
spectroscopy: Create a group product with _arc suffix.

This is to make the Science Archive wrapdr easier.  Also use
the standard form of naming rather than arc_<obsnum> for the
preview images.
spectroscopy: Modify the output file's suffix.

The online documentation says this should be _dbs not _std.

http://www.jach.hawaii.edu/UKIRT/instruments/cgs4/orac-dr/filetypes.html
This also brings it into line with the equivalent IFU primitive.
spectroscopy: Add PRODUCT header.

This was missed the first time because of the incorrect file suffix
corrected in Commit cf46c4c65fd4e.
spectroscopy: Tidy comments.
spectroscopy: Rename product to wrapdr friendly form.

Rather than std_<obsnum>... use g<i><utdate> form with a _standard
suffix to match the productID.
spectroscopy: Remodel the creation of the tagged product.

The aim is to obtain a group file of the form g<i><utdate>_<group>_arc
for the sciene-archive wrapdr script rather than Frm equivalent.  Use
a temp Grp file and let _TAG_AS_REPRESENTIVE do the most of the work,
passing it the arc as the input file and then request an update of the
Grp file.  The previews are derived from the suitably named product.
ifu: Remove tabs.
ifu: Correct $out being declared twice.
ifu: Create previews and PRODUCT keywords for the standard.
ifu: Only make the previews for the flux calibrated image.
ifu: Set the correct product name for the final spectrum.