add ImmediateContainer, Quantity and SimplyUnit, clarify Unit use

[odscjen]

closes #1343

draft until comment is agreed upon

[jpmckinney]

Just a couple comments on the Markdown. I'll review the schema when ready.

[odscjen]

Error
FAILED tests/test_schema_integrity.py::test_versioned_release_schema_is_in_sync - UserWarning: /definitions/Quantity/properties/value has unrecognized type ['string', 'number', 'null']

Quantity has been lifted straight from the medicine extension, I think the test may need updating

[jpmckinney]

Noting that I'll correct a "agreeemnts" typo and update the profile template via this PR before merging.

[jpmckinney]

I considered adding 'GDSN' to itemClassificationScheme.csv (similarly, we could add a code for the immediateContainer codelist from the medicine extension), but since we discourage units of presentation for the unit field, I think it makes sense to omit it.

[jpmckinney]

I made edits as described in commit messages. @duncandewhurst You might want to review commits individually, skipping the chore and ci commits.

[duncandewhurst]

I reviewed the individual commits and ran the pre-commit script to update the dereferenced release schema so that I could review the HTML documentation.

The changes look good, but the first sentence of the descriptions of Item.unit and Item.immediateContainer simply restate the title of the fields. Perhaps they could be improved? e.g.

• .unit: "The pricing unit" -> "The unit in which the pricing of the items is specified"
• .immediateContainer: "The immediate container for the item" -> "The innermost container in which the items are packaged"

Anyway, happy for you to merge if you think the descriptions are OK as-is.

[jpmckinney]

Good call - I'll reword to be more informative.