heterodyne: Have arguments to convey user-requested baseline regions.

This is applicable to spectra with weak emission and multiple
lines.  Some users prefer to specify where they expect no emission
rather than rely on automatic mode in MFITTREND to mask all the lines.

It adds a new BASELINE_REGIONS argument to ITERATIVE_GROUP_PRODUCTION_,
from which lower and upper bounds are passed to _REMOVE_BASELINE_
through new LOWER and UPPER arguments.  There is a corresponding
BASELINE_REGIONS recipe parameter.

Also included is a reordering to alphabetical of the METHOD/ORDER
arguments in _REMOVE_BASELINE_.  [This change could not be easily
disentangled from the new LOWER and UPPER arguments.]
heterodyne: Create new BASELINE_REGIONS recipe parameter.
heterodyne: Improve description and wrap long line in prologue.
heterodyne: Correction to an argument description.
heterodyne: Extend BASELINE_REGIONS to non-linearity tests.

If the user wants to define the locations of baseline, it seems silly
not to use them also for baseline non-linearity tests.  One could
detect BASELINE_REGIONS being set and override BASELINE_LINEWIDTH, but
for now I have added a further option for the latter recipe parameter,
namely "base" to choose the BASELINE_REGIONS.  As the two non-linearity
primitives mask the emission first, the complement of the
BASELINE_REGIONS must be formed before performing the QA tests.
heterodyne: Allow more flexibility in BASELINE_LINEARITY_LINEWIDTH recpar.
heterodyne: Document new and extended recipe parameters.

This comprise BASELINE_REGIONS and the "base" option for
BASELINE_LINEARITY_LINEWIDTH, which uses BASELINE_REGIONS.
heterodyne: Need to extend flexibility in BASELINE_LINEARITY_LINEWIDTH here too.