Add info and ok endpoint and refactor code (#360)

* Add info endpoint

Fixes #358

* Add ok endpoint too

* Try to please linter

* Woops, of course assert is not a function but a keyword

* ok does not require authentication

* Fix typo

* Remove code duplication by eliminating get_documents_by_type

It had the exact same implementation as get_documents except
that it added an extra argument.

* FIx linting errors

* Replace all validate_graph_type calls with proper types

Checking string constants each time is in my opinion not good.
We should use proper types for this like the GraphType enum.

I also made sure it is backwards compatible. Calling the functions
the old way will work fine.

* Remove unused function

_ref_extract was not called anywhere inside the codebase
and no docs even mention it. The tests don't call it as well.

* Add GraphType to higher level export

* Add class_frame test

This test case was not covered

* Fix type hint

* Use GraphType enum for good type checking

* import Schema

It was used for type hinting but not imported

* Remove schema import again to prevent circular import