New POWHEG gitlab repo

Dear ALICE POWHEG Team,

POWHEG has transitioned from svn to git recently. We are using https://gitlab.com/POWHEG-BOX/ now (more info on using it's submodules here https://powhegbox.mib.infn.it/). If you have put some changes/patches in your clone of the svn repo here and they could be of general interest for all POWHEG users please feel free to upstream/MR them there.

Cheers,
APN