tot_tours Hidden in Non-Mandatory Tour Frequency Code

[dhensle]

A variable named tot_tours is hard-coded in the non-mandatory tour frequency model: https://github.com/ActivitySim/activitysim/blob/main/activitysim/abm/models/non_mandatory_tour_frequency.py#L159.
This should really be exposed to the user and listed in the alternatives file instead.

[JoeJimFlood]

@dhensle Is what I did in #661 along the lines with what you were thinking? I am wondering if we should add a check that the total tours is actually the total number of nonmandatory tours for each alternative and raise an AssertionError if that's the case.