Import mimic-code as a submodule

[alistairewj]

Importing mimic-code (https://github.com/MIT-LCP/mimic-code) as a submodule would allow direct loading of queries from that repo instead of storing a duplicate in this one (e.g. angus.sql is a duplicate here). It would also facilitate creation of a function which created all the materialized views of severity scores which are needed for the analysis.