New feature: LHE Reader can read proposed LHE weights tags (standard in MG5)

The LHESource has been modified to be able to read weights with the structure defined here:
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Reweight

This was tested on 50k events containing embedded weights, and 50k events with no weights.
The reader shows no faulty behavior in the no-weights case, and works to arbitrarily high numbers of weights.

The weights definition header is currently stored in the same way as other headers in the LHERunInfoProduct in CMS.