Add total MFD for source tree to Models serialization classes
This may involve adding a totalMfd() method to RuptureSet. Most concrete RuptureSet implementations alrady compute the 'totalMfd' even if it is not used directly in hazard. The totalMfds across all rupture sets i a source tree then need ot be scaled by their weight and summed in the same way that the logic tree of MFDs in a RuptureSet are combined/collapsed.