Extended mean_recorded_value, added min_recorded_value and max_recorded_value

• Extended mean_recorded_value to support querying for mean recorded value across a full date range, as well as the most recent day of measurement.
  • Docs: cohortextractor.patients.mean_recorded_value
• Added min_recorded_value to support querying for minimum recorded value.
  • Docs: cohortextractor.patients.min_recorded_value
• Added max_recorded_value to support querying for maximum recorded value.
  • Docs: cohortextractor.patients.max_recorded_value

Pull request: opensafely-core/cohort-extractor#726