PICARD: trim the map first to avoid processing unnecessary pixels

When processing large files (larger than pong3600) made with 1"
pixels, it's a waste of time to calibrate that whole file and then
trim it to 180" across. Trimming it first is much more efficient.
PICARD: protect against another divide-by-zero bug

If the ITC obstype is not known, the rms returned by calcrms() is
undefined, which leads to a div-by-zero error when calculating the
ratio of the map to ITC rms values. Set an initial value of NaN and
proceed if the ITC rms is defined and non-zero.