Refactor human input to a separate module

[jakebeal]

The places where the script gets input from a human are currently inline with the logic code, which makes it difficult to test or to change input source in the future (e.g., from command line to Jupyter or web interface).

We should refactor those requests for input into a separate module, which gets called to get input. That module can then be readily reconfigured to provide options for alternate inputs if desired.