Moving away from oneOf

[glenrobson]

Fixes #154

[glenrobson]

This is quiet a large change to move away from oneOf in the schema to if/then/else. It provides much better error reporting than oneOf which prints a generic:

"is not valid under any of the given schemas"

without any context as it doesn't know which oneOf is closest to the one you want. This means I can get rid of all of the custom error diagnosis in error_processor.py and rely on Jsonschema to produce a better error message.

I've run out of time to complete this pull request but still to do:

• check if/then/else works with iiif-prezi3
• move failing fixtures to /fixtures/3/broken/
• check error messages of broken (both path + title and description)
• remove redundant code in schemavalidator.py and the file error_processor.py
• remove any other oneOfs in the schema
• look at moving placeholder and accompanying canvas to being recursive
• move schema tests into its own file
• read all valid manifests and check

[glenrobson]

So Mike tested and unfortunately pydantic-codegen doesn't support if / then / else. But it looks like this use of if then else is what is intended: (see json schema issue: json-schema-org/json-schema-spec#31). For pydantic we can downgrade the if then else block to the following form:

{
  "if": {"$ref": "condition" },
  "then": {"$ref": "schema1"},
  "else": {"$ref": "schema2"}
}

to:

{
  "anyOf": [
    { "allOf": [ {"$ref": "condition" }, {"$ref": "schema1"} ] },
    { "allOf": [ {"not": {"$ref": "condition" } }, {"$ref": "schema2"} ] }
  ]
}

From json-schema-org/json-schema-spec#180