Introduce model_locator to resolve model files from env in a consistant way

[andreas-bauer]

This PR introduces src/model_locator.py to resolve model file paths from env vars in a better way. I encountered problems because not every Python file does the is-in-dev-container check and therefore fails in some environments. To avoid further headaches, this model_locator.py is the single source of truth when resolving the model file paths.

The model paths can now be resolved using model_locator.labeling() or model_locator.classification().

Further, this PR changes the behavior for the DEV_CONTAINER env. The new behavior is as follows:

• If .env file and the targeted env exist, it will use it directly
• Otherwise, _DEV will be appended to the env name and then resolved

This change in behavior makes the DEV_CONTAINER env obsolete and should be removed from the codebase in further PRs.

[andreas-bauer]

@JulianFrattini PR #52 needs to be merged first.