Bug Fix for xPPSoffset importing, save corrections

This patch fixes an error introduced in the previous series of patches
in which the xPPSOffset data file import had a different format (fewer
columns).  The import routine is very strict and I had temporarily
modified it, but failed to undo the change before committing.

This patch also introduces a feature/option to allow saving of the
calculated dT corrected for xPPSoffset values.  See --outputPrefix.

Finally, there are several enhancements to the plotting.  Some plots now
show grid lines and have better spacing (under some conditions).  Plus,
some protections for edge conditions.
Merge branch 'dev' into public

Important update.  Bug fixes (restore newer xppsoffset format importing)
and useful new feature to save calculated dT adjusted with xPPSOffset
values, and other updates.