WIP: modernization

[blakerosenthal]

Plan:

• [x] update to Python 3
• [x] use ActivitySim's pipeline for managing steps
• [x] use ActivitySim's model configuration structure, using YAML files instead of hard-coded settings
• [x] separate incremental_demand, benefits, and assign into model steps
• [x] support sqlite inputs and outputs for trip tables
• [x] support CSV inputs and outputs for trip tables
• [x] support inputs and outputs for skims
• [x] create 25-zone example data
• [x] support zone-indexed skim inputs
• [x] remove hard-coded column indices
• [x] update documentation
• [ ] add tests