remove HSC-specific forced photometry (DM-620)

These files were originally used in combination with hscMosaic
(the "stack" data product), which is no longer used.  This is
to prevent confusion with the forcedPhot* modules in pipe_tasks.
Merge branch 'tickets/DM-620'
stack: consolidate try/except in new context manager

PbsCmdLineTask now provides a "logOperation" method that
provides a context manager that logs the start and finish
of the operation, and optionally catches exceptions.
Merge branch 'tickets/DM-662'
stack: fix up tract,patch handling

Last change broke handling when no tract was specified.  Divided the
code more logically.
stack: add parallelised determination of non-empty tracts

Iterating over 10^4 tracts (e.g., for a RingsSkymap) can be very slow.
This provides a parallel determination of which tracts are non-empty,
so only those can be iterated over.
SConstruct: remove Mercurial
adapt to genericised batch system changes in hscPipeBase

The tiger cluster will soon be entirely slurm.  This adapts
to DM-741 changes in hscPipeBase that provide a generic batch
system instead of PBS-specific.