Add Python tests and GitHub actions workflow

[duncandewhurst]

• Add a GitHub actions workflow to run tests for:
  • Empty JSON files
  • Invalid JSON files
  • Incorrectly formatted JSON files
  • Invalid JSON Schema files
  • Invalid and incorrectly formatted CSV files
• Add developer documentation that describes what to do if the tests fail
• Indent JSON files
• Fix broken refs in schema (where needed, I added empty definitions. I assume that we'll fix these as part of other planned schema changes)
• Fix incorrectly formatted CSV files

As noted below, some JSON Schema checks are commented out until the issues that they relate to have been addressed:

rdl-standard/tests/test_schema.py

Lines 36 to 56 in 068112d

	def validate_json_schema(path, name, data, schema):
	errors = 0
	errors += validate_schema(path, data, validator)
	errors += validate_array_items(path, data)
	errors += validate_items_type(path, data)
	# Codelist fields are not yet implemented
	# errors += validate_codelist_enum(path, data)
	errors += validate_merge_properties(path, data)
	errors += validate_ref(path, data)
	# Titles and descriptions are not yet added
	# errors += validate_metadata_presence(path, data)
	errors += validate_object_id(path, jsonref.replace_refs(data))
	errors += validate_null_type(path, data, no_null=True)
	# Here, we don't add to `errors`, in order to not count these warnings as errors.
	validate_deep_properties(path, data)

[duncandewhurst]

Actually, following open-contracting/jscc#8 (comment), I can use the main JSCC repo instead of the fork that I created.

[duncandewhurst]

@stufraser1 please could you review this PR? It will be useful to have these tests running on the dev branch before we start merging schema PRs.