Add total MFD to source logic tree service

In addition to returning branches and weights, also return total combined MFD for each SourceTree.

added Services label

mentioned in issue #525

added ERF label

changed milestone to %NSHM Hazard Tool - Sprint 2

Total MFD was added to serialized object from nshmp-lib Models class.

See nshmp-lib!288 (merged)

closed