MIMIC-IV MySQL build script update

[alistairewj]

This is from a re-run of csv2mysql from @pszolovits - looks to add more info in comments, modify data types modestly (e.g. unsigned int to int, and some small ints to medium ints) - some seem like they will result in performance improvements (e.g. a TEXT field converted to VARCHAR(255) in emar_detail).

I've also added a GitHub action for building the demo in mysql but it will likely fail since I just tried to write it blindly.. so PR is still a draft.