When a logit model is solved, add the ability to get the logsum in addition to the choice

[bstabler]

This is needed for upstream models and model estimation

[bstabler]

Task Spec

If you add an entry to a destination choice or mode choice model settings specifying a logsum column name, it adds one in addition to the choice. So for example if you add this:

DEST_CHOICE_LOGSUM_COLUMN_NAME: school_taz_logsum

Then persons will get an additional column with the logsum value. We're adding this feature to the tour destination choice and tour mode choice models as an example of how to use this functionality.