heterodyne: Count the number of spectra in a time-series observation.

Factorise existing code into its own primtive.  At present it
does not check that the third axis is indeed the temporal axis.
heterodyne: Use the number of spectra rather than the mode to decide applicability.

Also take out factorised code and document minimum number.
heterodyne: Add MIN_SPECTRA argument.

Use this argument to control the minimum number of spectra hence
applicability of this primitive, rather than decide by observing mode.
heterodyne: Don't use the observing mode to decide applicability.

Use the number of spectra or elements instead.  Document the minimum
number of spectra/elements too.
heterodyne: Reorder a recipe parameter.