Finish switching MCS header construction from gen2 header to gen2 keywords

The mcsActor fetches the gen2 telescope keywords, and uses them to construct the WCS cards. But it still actually fetches and uses the pickled header from the gen2Actor for eveyrthing else. This should be fixed.

The only time-consuming part (and the requirement for this ticket) is putting together functions to construct the required time cards. Separate ticket.