Move logic out of __init__.py

[tompollard]

As discussed in Medical-Event-Data-Standard/meds#3, __init__.py is usually just a place for putting imports and other bits and pieces to help with packaging.

https://github.com/Medical-Event-Data-Standard/meds_etl/blob/main/src/meds_etl/mimic/__init__.py currently contains most of the logic for transforming MIMIC to MEDS. We should move the code out of __init__.py and into a module.