OMOP CDM docs

[Iain-S]

Work in progress.

[Iain-S]

Today, @cptanalatriste and I made changes / additions until we were able to successfully run make-tables, make-generators, create-tables, create-generators and create-data on the cms_desynpuf_sample_53_large schema:

• could be in a separate PR truncated String col types to account for a couple of VARCHAR(1)s
• made a custom config and generators file to account for the combined pk & fk of death.person_id
• could be in a separate PR returned None (IE no rows) from ColumnValueProvider rather than erroring if there are no rows in the table using getattr(...,None)

Still to do:

• add tests for the two code changes
• optionally split the two code changes into a separate PR as they aren't OMOP-specific
• rebase on main / merge in changes from main
• in the example omop config, we commented out the cohort def and attribute def tables so that they aren't treated as vocab. we should either explain this in the docs or add some data to those tables and reinstate them as vocabs (I'd prefer the latter)

[cptanalatriste]

@Iain-S I will create new issues for the suggested PRs and work on them.

[cptanalatriste]

Looks good to me. Well done!