Schema endpoint validation

[walkingtowork]

@rmutter @baylee https://github.com/yeti/signals/issues/38

• Added path validation to the --datamodels flag in main.py
• Added validate_apis_and_objects function to schema.py
  • Loop over the endpoints for each url in the schema, then search the objects dict for matches of parameters, requests, and responses, and throw an error if a match isn't found.

[rmutter]

@walkingtowork some comments + tests failed