Avoid unnecessary introspection for MVAComputer

NOTE:  Do not merge this pull request into CMSSW_7_3_ROOT6_X, as there will be merge conflicts.
This pull request is for code commonality between CMSSW for ROOT 5 and ROOT 6.
In the ROOT6 IB, unnecessary introspection has been removed from MVAComputer, to save memory and increase performance, as this particular use of introspection causes run-time header parsing.
This pull request makes the equivalent change in the CMSSW ROOT 5 IB, for code commonality.
The introspection was used to invoke the copy constructor of classes derived from VarProcessor.
In less than an hour, I was able to replace this ugly introspection with a virtual clone() function, which can be called directly.