New Pythia8 Hooks for showering Powheg events

Hi @ewelina,

I am merging three new Pythia8 UserHooks.

• PowhegHooksBB4Ltms, which replaces PowhegHooksBB4L, to be used for showering BB4L events (AGENE-1941). I have validated its implementation against the standalone Pythia8 and against the author's events.

• TopRecoilHook, to force the recoil of subleading emissions off a top decay to recoil against the top resonance itself. Also here I checked I get in athena the same output as the Pythia8 standalone code.

• PowhegV_EW, to perform a simultaneous veto of QED and QCD emissions, needed for Powheg processes with NLO EW (AGENE-1807). This latter hook was prepared by @gpanizzo, and I have only tested it does run in athena. While more validation will be needed it would still be useful to have it in a release. The code itself prints out a warning that it should not be used without further validation.

It may be good if @gpanizzo or one of the other experts could have one last look at these.

cheers, Simone