heterodyne: Add minimum spectra argument (MIN_SPECTRA).

Small numbers of spectra either cause certain tasks like FINDBACK or
FIXSTEPS to abort with an error, and there needs to be sufficient
spectra to define a norm for non-linearity above which anomalous
spectra can be rejected.  For now set the default to 50 spectra, and
see how it works in practice.  There is a case to make the threshold
somewhat larger.
heterodyne: Protect FINDBACK from too few data points.

This is in case MIN_SPECTRA is set lower than 50.

This addresses fault 20121105.004 (latest reports).
heterodyne: Add two arguments pertaining to detection of spectra with ringing.

Also added a a paragraph about ringing to the description.
heterodyne: Add two recipe parameters to control filtering of high-frequency ringing.

Completes the work that began with 05b5864e9a0d28c48e.