Various schema updates

[javh]

Fixes #677 Fixes #679 Fixes #686

1. Set enum fields in Germline schemas to nullable: true. (#663)
2. Set _id fields in Germline schemas to identifier: true. (#656)
3. Added "IGI" to all locus fields. (#671)
4. Added quotes around values for various string enums.
5. Minor formatting issues. (#659)

[javh]

From the call:

• Add identifier: true to Acknowledgement.acknowledgement_id, Cell.cell_id, CellExpression.expression_id, Receptor. receptor_id, Clone.clone_id, Tree.tree_id, RepertoireGroup.repertoire_group_id
• CellExpression.reperotire_id, is misspelled in required.
• Added change for #670 to this PR.

[javh]

From the call:

• Need to address identifier issues from prior call notes.
• nullable has been addressed in other PRs (by @williamdlees), so we need to defer to those changes instead of those in this PR.
• Check for snake_case in all enums. Enforce snake_case instead of requiring quotes around enums, except in cases of currently in-use fields, which will require a version bump to change the enum values (make a ticket for v1.5.0). Require quotes for in-use fields with spaces in enum values.
• Resolve merge conflicts, propagate schema to /lang, and merge.

[javh]

Addressed issues above except for the enum snake_case topic, which is moved to #673.