PowhegControl: add directphoton process

Requested here: https://its.cern.ch/jira/browse/AGENE-1724

These changes add the Powheg event generation; the matching of the parton shower in the presence of both QCD and QED radiation at the Les-Houches-level still needs to be implemented on the Pythia side (and then set up on the PowhegControl side).

The process has so far only been tested with a very coarse integration to check that it works from a technical perspective. A heavier integration is in progress, but since this process will anyway require some careful physics validation, I think it's good to merge the technical setup already now.