add non-mandatory tour frequency extension

[bstabler]

The individual non-mandatory tour frequency model operates in two stages:

• First, a choice is made using a random utility model between combinations of tours containing zero, one, and two or more escort tours, and between zero and one or more tours of each other purpose.
• Second, up to two additional tours of each purpose are added according to fixed extension probabilities.

The first stage was implemented in the early stages of the project and the second stage has yet to be implemented.