minor bug fix in base.py Model class

nVSamp must be an int, not a float

relabeled log10(vcirc) to be shorter

corrected origPars for model A to use log10(vCirc)
clean up modelConstraintPlot obs definition and parallelize

Replace procedural definitions of positions and values of observables
with call to ensemble.makeObs (which also adds missing assignment of
model.kernel for case where convOpt is not None).

Add nThreads keyword in calls to fitObs.