[Run2_2017] Change the type for the PDF weights branches

Created by: aperloff

Change the PDFWeights, PSweights, and ScaleWeights branchest from vector to vector. This reduces the size of the TreeMaker ntuples by about 7.7%. It changes the PDFWeights from being the second largest branch, behind Jets, to the 5th largest branch. This change is something we've said we'd try for a long time. Is this change acceptable from a precision standpoint, ignoring the space saving benefits?