heterodyne: Access recipe parameters for the percentage trim.

This is an addition to 20a126ebd055a63e.  It wasn't included originally
because _REMOVE_FREQUENCY_ENDS_ADVANCED_ invokes this primitive.
However, I found that this primitive is accessed in the continuum
recipes.
heterodyne: Set default percentage trimming according to the backend.

The DAS data appear to need at least 7% to remove noise spikes, where
the HARP data require around 3%.  For DAS I set to default to an
empirical 7.5% to remove spikes whilst retaining as much good data as
possible.  2.75% is the normal default for ACSIS recipes.

A dynamic solution would better measuring the noise excluding elements
whose local noise exceeds some threshold factor above the global noise
level.  This should suffice for now so that Jan can continue his
whole-night tests.
heterodyne: Remove explicit trimming percentages from recipes.

This is to enable appropriate trim percentages for both ACSIS and DAS
using the defaults set in Commit 8afb8189949a5a3bbb2.  The QA variants
had different low and high percentages.  In case there is a reason for
that, I have not modified those.
heterodyne: Protext against absent LO frequencies in the headers.