docs: Use internal headers not the pre-translated names.
heterodyne: Correct the primitive name in error messages.
heterodyne: Extend the check for a time-axis label.

There must surely be a better way using the domain.
heterodyne: Try a different approach to see if the final axis has a TIME Domain.
heterodyne: Allow for degenerate axes.
heterodyne: Validate statistics before using them to threshold.
heterodyne: Improve the collapse steps.

- Only collapse non-degenerate receptor axis.  This caters for the case
where a single receptor has been extracted already.
- Factorize time-axis collapse out of the first-loop block.
- Choose the method for the collapse of the receptors depending on the
number of receptors.