Rewrite/refactor top-level driver code

[ethanwhite]

analysis/fit-predict.R is the current top-level driver code. It handles different kinds of models differently and has no meaningful structure or documentation. Key changes to made to this code include:

• Abstract how models are handled so that all models can be iterated over together
• Extract handling of different setups/data for forecasting vs hindcasting into one or more functions
• Generalize handling of parallel code
• Document
• Modularize

Alternatively we could return to the associated code in forecast-bbs-core.R as the starting point.

[ethanwhite]

My general feeling after a few minutes with this is that we probably want to rewrite the top-level code from scratch while using fit-predict.R as a guide to make sure that the associated special cases don't get lost.