Add distributable single-command runnables for MIMIC-IV and eICU

[mmcdermott]

This entails both:

• We should have a secondary multi-stage runner script that runs a full pipeline via a single command. #55
• Incorporate downloading the requisite files into distributed code somehow.

For now, we leave "make pre-MEDS pip installable somehow" out of scope, and let our "distribution" pipeline be "run a shell command which downloads some files" instead a proper pip installation.