add originalXWGTUP() to LHEEvent and LHEEventProduct

Add some book-keeping information to LHEEvent and LHEEventProduct so that reweighting is less cumbersome.

In the MG5 scheme, users can now do raw_ME_ratio[i] = eventInfo->weights()[i]/eventInfo->originalXWGTUP();

To get the raw matrix element ratio.

I have checked that this is stored correctly by writing out the LHE file that was read in. 
The XWGTUP parameters matched exactly.