Add an IPeaksWorkspaceProperty to Python

and put back the usage example for PredictFractionalPeaks that was failing.
It failed because Python didn't understand the type of output
it was trying to give back.
Refs #9642
Add required boost:: qualifier for dynamic cast

Refs #9642
Merge remote-tracking branch 'origin/bugfix/9642_py_except_predict_fractional'