Various fixes and addition of quality__meta_date

[mikix]

• Add a new meta/ folder that makes the metadata tables
• Add quality__meta_date that tries to find outer bounds of dates among all the data (excluding error and planned-encounter rows). This is basically a mini version of the q_date_recent metric, which we might add in the future.
• When checking Patient.gender for the US Core profile, actually inspect the string and only accept the four spec values.
• Refactor get_date into get_date_string and get_date, to allow doing SQL logic on the date value.
• In q_ref_target_valid, support DocumentReferences with a low schema (i.e. without context.encounter) - which isn't super realistic, but still.
• Add CVX and NDC to the list of systems we provide a user friendly short name for, in c_term_coverage.