Onboardapis

\* Fixed MRO for ``FlixTainment``

\+ Added ``NotImplementedInAPIError`` that is raised every time a functionality is requested by the user that is not covered by the providers API
\+ Added the ``IncompleteVehicleMixin`` and ``IncompleteTrainMixin`` mixin classes, that will raise a ``NotImplementedInAPIError`` on not implemented abstract methods
\+ Added the ``DummyDataConnector`` that is now the default ``DataConnector`` connected to a ``Train`` object
\+ Added support for FlixTrains ``FlixTainment`` API

Known issues:
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Can't use ``FlixTainment`` because of wrong MRO - fixed in v1.3.1

\* TLS can't be disabled for a whole ``DataConnector`` anymore
\* Reorganized and optimized parts of the backend code

\+ Added more type hints

\* Fixed ``JSONDataConnector`` adds an _application/json_ header
\* Fixed ``Station().connections`` for ``ICEPortal``