copy over PileupReweighting from 21.2

I had started out by sweeping changes from 21.2 to master, but that turned out to be a big mess, given that a lot of changes were applied in either branch and many in both. So this is now just removing the package as it was in master and copying over the package as it is in 21.2 right now, followed by some cleanup of obvious places where development work was undone or where what was in 21.2 was clearly out of line with current best practices.

As with all sweeps/copies from another branch, this should only receive a "lite review", i.e. only flag real problems/bugs, but skip any style comments or suggestions for improvements.