added a check to multiDimFit.py for existing output file and option to overwrite it

added a check to multiDimFit.py whether the final output file
(produced by hadd) already exists BEFORE running anything (to
avoid that a lot of time is spent running and then hadd fails).

Also added an option --overwrite to force overwriting of
the output file (which disables this check and passes
an option to hadd to force overwrite the output file).