Multi err modes for PVT and cleanup

 - change measGrpID to cell array of string IDs
 - initPVTdata now initializes nearly empty PVTdata struc
 - data is loaded into PVTdata using 3 possible methods, reflecting diff
error modes
   - setPVTdataTot - total error user defined
   - setPVTdataStd - total error propagated from P,V,T
   - setPVTdataMark - total error propagated from Vmark,V,T accounting
for correlations
 - Total propagated error now calculated in updatePVTdata, specific for
each error mode
 - update initPVTeval to generalize multiple measGrpIDs. The code runs
but it is not yet used in the errorModel
 - devel includes scripts for fitting Tange data and testing validity of
normal approx (testCostFun)
 - fix bug in getFreeParams, now allows zero covariance for fixed params
 - fitHotCompressData returns PressTotFun and opt for convenience