BayDAG Contribution #9: Mode Choice Logsum Extraction

[dhensle]

Two types of logsums are calculated in the location choice model: a mode choice logsum (where the utility is summed across all modes from the origin to the alternative destination) and the destination choice logsum (where the utility is summed across all destinations and includes the mode choice logsum in the utility calculation). The current model only returns the destination choice logsum. This PR allows the user to also fetch the mode choice logsum of the selected alternative so it can be used in downstream models.

Required for SANDAG ABM3 production? -- yes