Mutually exclusive fields in JSON schema

[felixfontein]

There are multiple places in the schema where mutual exclusion is checked by code. We would eventually like this mutual exclusion to show up in the JSON schema.

@abadger thinks this is the pydantic feature which will allow us to add an xor to the json schema: https://pydantic-docs.helpmanual.io/usage/schema/#schema-customization

Whoever does implement it will also need to know a little about json-schema. @abadger thinks this is the heart of what we'll need to output: http://json-schema.org/understanding-json-schema/reference/combining.html#oneof