heterodyne: Add absolute threshold option to receptor non-linearity.

Some of the early GBS data have many and large non-linearities. Using
kappa-sigma clipping can leave receptors with non-linearity
coefficients of 0.2, when the goal is more like <0.05 and certainly
below 0.1.  This change provides an alternative fixed threshold to
eliminate all wobbly baselines above this level.  The default may be
too conservative at 0.10.  I'm using 0.08 for GBS.  Below that it's a
trade-off of losing data and signal-to-noise versus some subtle striping
in the reduced cubes.

Create a new recipe parameter for this BASELINE_LINEARITY_RMS_THRESHOLD.
heterodyne: Add reserve reference receptor.

The regular reference receptor, H05, can sometimes suffer from bad QA
or non-linear baseline interference.  Therefore, have a reserve
receptor to normalize against.

Add the RESERVE_REFERENCE argument with the default H10 being another
central receptor and the one used in the Richer, Curtis, & Buckle
(2009) Perseus paper.  Empirical data suggests that H10 is rarely
affected by interference.  However, it may bias the flux
normalisation.  The trade-off is to not flat field an observation if
the reference receptor is invalid giving relative errors, or use the
reserve affecting the absolute values.  There could be an argument &
recipe parameter to control this choice.
heterodyne: Document the recipe parameter for MINRMS argument.
heterodyne: Another tiredness typo.

Listen to the voice!  I should have stashed this primitive until I
could test todays's trivial (ha-ha) additions.  The previous
uncommitted ones were fine.  At least it compiles now...