Clean up and generalize the summary tables

[smmaurer]

We should clean up the summary table functionality, probably creating a general-purpose function to generate tables. Currently this is part of the MNL codebase: mnl.py#L336-L479

Cleaning it up would streamline the codebase, lay groundwork for additional model types, and allow easier extension of the summary table functionality (e.g. as requested in UrbanSim Templates issues #24, #48, and #59).