Fix lingering issues with the "Big Island" bugfix

Correct issue in omni_grs_maskspec.pro where a single-element peak is the
brightest, yet is ignored because the "saddle-finding" algorithm requires at
least two points in the peak.  This causes the unintended result of no peak
being fit (since the algorithm loops until it finds the brightest peak.  This
has been corrected by only applying the saddle-finding algorithm to peaks with
2+ elements, and otherwise using single-element peaks as-is.

Also, add !EXCEPT=0 statements to silent the underflow math errors.  Still not
sure where they are coming from, but it doesn't really matter.

	modified:   subroutines/omni_grs_maskspec.pro