save post-fit snapshot in workspace for MultiDimFit and add ability to l...

...oad named snapshot from the command line

Usage example:
text2workspace.py hgg_datacard_mva_8TeV_bernsteins.txt -m 125 -P HiggsAnalysis.CombinedLimit.PhysicsModel:floatingHiggsMass --PO higgsMassRange=120,130 -o testmass.root
combine -m 123 -M MultiDimFit --saveWorkspace -n teststep1 testmass.root  --verbose 9
combine -m 123 -M MaxLikelihoodFit -d higgsCombineteststep1.MultiDimFit.mH123.root -w w --snapshotName "MultiDimFit" -n teststep2  --verbose 9 --freezeNuisances MH,lumi_8TeV

In the above example, one sets up and performs the floating mu,MH fit

Then for the final step, the output is loaded and a new fit performed with MH and lumi_8TeV frozen to their best-fit values from the previous fit.  (Any nuisance or other floating parameter in the workspace or physics model can be specified in freezeNuisances and things should behave properly)