Fix the datetime in synthetic mimic-iii data

[zzachw]

Current API will throw an error with datetime parsing if you try to load notevents table from https://storage.googleapis.com/pyhealth/Synthetic_MIMIC-III. This is because the noteevents in synthetic mimic-iii data has a different datetime format than the actual mimic iii. We should fix the datetime in synthetic mimic-iii data.

[jhnwu3]

Should be mostly fixed, but we still need to merge the PRs
#407
#403
#380