PICARD: even better test for supported ITC-supported pong sizes!

Thanks to Graham for suggesting this in a comment on b1a8c73615. This
is a future-proof method and doesn't rely on keeping this primitive up
to date with the latest rules for acceptable pongs (sic).
PICARD: calculate stats for signal coadd as well

...and not just the matched-filtered version.
SCUBA2: set new value *after* warning message

Warnings make so much more sense if you set the new value *after* the
warning...
PICARD: check for bad values and set to NaN