Improve const protection in PRW tool
This is a follow-up to !59646 (merged), which allowed running pileup reweighting on files containing partial pre-computed information. There are a few additional instances of the PRW tool unnecessarily accessing event decorations via non-const reference that the first MR didn't catch. This fixes those (by substituting auxdecor
-> auxdataConst
) to avoid further problems with const-correctness.