Open requested to merge LHCBGAUSS-471.LbPowheg into master
Make necessary changes to be able to use PowHeg-Box again within Gauss.
This required to migrate LbPowheg to the new plugin structure that was introduced in Pythia 8.209 for PowhegBox and to get dynamic plugin libraries.
This MR replaces !96 (closed), !559 (closed) and !536 (closed)
It will require a first release of