update & document source config paths for dataset compilation

[rlskoeser]

The compile script has some defaults for paths for the data ingredients:

• source_excerpt_data
• source_ppa_metadata
• maybe also poem metadata

These should be documented in the sample config file and defaults should move to the get_config method.

Add unit tests for the compile dataset script & methods

[rlskoeser]

I worked on this some, but because I was bouncing between branches and this wasn't a priority, I wasn't able to complete it. I'm increasing the estimate from 1 to 2 because it's more complicated than I thought — configuration logic, defaults, and checks are distributed between a few different files currently, which should be consolidated and made consistent.

Completing this is still important for others to be able to work with the data and to make the compilation reproducible; I'll move to the ice box for now.