implement nested logit

[bstabler]

this is a place holder for task 7 nested logit comments, issues, discussion.

[bstabler]

SANDAG's auto ownership model is nested. In order to make it easier for modelers to switch from MNL to NL for example, we will improve ActivitySim's sub-model definition requirements. Here is the basic plan for improvements:

• each sub-model will always look for a settings yaml
• each model will get a LOGITTYPE setting with the possible current options of MNL and NL
• the existing sub-model code and documentation will be updated as a result
• we will move the accessibilities locals to an accessibilities.yaml file as well

For testing, we will implement the SANDAG nesting structure with the existing example. The SANDAG auto ownership model nesting structure is:

• 0cars is in a nest by itself
• 1car to 4+cars is in a nest
• the nesting coefficient is 0.68

[bstabler]

done