Multi Meas Grp enabled (general and PVT)

 - fitErrModResidMultiGrp has been added to handle multiple measurement
groups by cycling through and individually fitting params for each
 - fitErrModResid has been simplified to remove all mention of multiple
meas groups
 - fitErrModPVT allows fitting multiple meas groups
 - tests for everything EXCEPT....

 NOTE: test in fitLibTest for MultiGrp works and passes visual
inspection, but the test assertions need fixing