Force trigger mode to +TIME
Force +TIME mode for C++ version, too

Adds the same changes to C++ version as previous commit.
Changed exit hook to be more graceful

Exiting comSR620 occurs by sd_hook() which is attched to SIGINT.  But, I
have noticed that the TIC is always confused after doing this.  For
exmaple, the next run of comSR620 will always fail with "not a TIC!",
but then the next run will work, but often, the next run will be plaged
with random zero measurements.  I think the TIC is getting confused.
So, I've updated the hook with a few SR620 commands which stop whatever
measurement is active and stops any auto-measurement that is active.
I also replaced some planned exits that called the hook with direct
exit() calls if I thought it was safe.
added periodic recalibration code
fixed silly syntax error...
Beta version of auto-calibration code

Several adjustments since first commit on this topic.  Still needs
testing.
Merge branch 'calib' into dev

Much of this has been tested, so I'm merging into dev.  But, the new
function to restart measurements is untested.
Auto-Calibration COMPLETE

This commit includes the final changes that implement the
auto-calibration procedure.  This version has been thoroughly tested
with calibration interval at 5 and 10 minutes.