Added refToPSet_ ability in python PSet

The python PSet class is now capable of containing a lazy evaluated reference to a PSet at the highest level of the Process. This is done by using a parameter named refToPSet_ which takes a cms.string with the label of the top level PSet to which one wants to reference. The reference does not happen until we convert the python PSet into the C++ ParameterSet. At that time a copy to the reference is used to create the C++ ParameterSet.
Merge pull request #3072 from Dr15Jones/addRefToPSet_

FWCore/ParameterSet -- Added refToPSet_ ability in python PSet