Add Zj MiNNLO interface to PowhegControl
This MR is mostly based on !29425 (closed) (containing the work done by @amoroso — thank you!). It fixes the merge conflicts with the target branch 21.6 and contains a few additional minor improvements to make the new interface more conform to the standard.
MR !29425 (closed) can be closed without merging after this MR is accepted, because all the changes are also included here.
Edited by Stefan Richter