Include PDF Varied Event Weights in Output SPODS Trees

Include event weights with each PDF varied as specified inthe PDF4LHC report: http://arxiv.org/abs/1101.0536
Also provide a central value and +/- 1 sigma weights between the various PDFs.

Needs to be an optional feature in the cfg file, and also with optional linkage of LHAPDF during compilation. Display a warning if it is configured to run in the cfg file, but not enabled during compilation.