general: Mask sections along a single axis.

I wanted a primitive to mask a list of emission regions.
heterodyne/_MASK_BASELINE_REGIONS_ only worked for the third axis of a
cube.  It's not clear how it worked in the context it was using of
velocitiies with pixel co-ordinates along the spatial axes.

This new primitive attempts to be more general in both the
dimensionality of the data and the axis to mask.  Hence I put in
general/ rather than heterodyne/.  It essentially creates RECT ARD
specifications and invokes ARDMASK.  Note that at present it requires
the referenced ranges to be specified in pixel co-ordinates.  Masking
can be within or beyond the supllied ranges.
heterodyne: Fix typo.
heterodyne: Permits a list of regions to exclude from non-linearity tests.

Andrew Rigby requested this feature, something I had considered doing
anyway for user processing in conjunction with the automatic detection
of the emission needed at CADC.

It calls the new _MASK_SINGLE_AXIS_REGIONS_ primitive (af22a82200d7).

Although it is no long a single linewidth, the LINEWIDTH name in the
recipe parameter and argument  is retained for compatibility with
existing recipe-parameter files and invocations in recipes.
heterodyne: Report a list of non-linearity emission regions.

This is related to 359ae3e1b5f119.
heteroedyne: Make consistent with the other steering files.

Although these only currently access numeric values, I had in mind to
have an automatic option using the noise.  In 20a126ebd055 the others
have the uc(), so be consistent in time for a non-numeric value.
heterodyne: Tidy some headings.
heterodyne: Add a link and fix typo.