Reduce function redefinitions with on-demand systematics

This should not redefine the whole helper method for every jet kinematic variation (when using on-demand calculation of those - with postprocessed the branch name is different, so this is done automatically).

There should be a gain in memory and (JIT) compile time, runtime speed should be the same.

WIP because more testing and validation is needed (and this is based on the https://gitlab.cern.ch/cp3-cms/bamboo/-/merge_requests/158 branch rebased after merging that one) -> seems to work fine