Provide constructor for eic::VectorXYZT from dd4pod::VectorXYZ and double

Creating an eic::VectorXYZT fourvector from a dd4pod mcparticles entry requires individual x,y,z setters. Ideally, we'd have

    for (const auto& p : mcparts) {
      if (p.genStatus() == 4 && p.pdgID() == 11) {
        // Incoming electron
        eic::VectorXYZT ei(p.ps(), p.E());
        break;
      }
    }