Ensure all produced files from tests are in a temp dir

[CasperWA]

Description:

Closes #220.

Add pytest fixtures for loading EMMO, as this is done in several tests.
Add fixture for location to repo_dir, instantiating it only once per pytest session.
Add a yield-fixture for creating a temporary directory that lives for the lifetime of the test function.

This removes the possibility to check through the generated files from the tests, keeping the unit tests being just that: unit tests.
However, I guess the generated files should be tested more now?

In short, the unit tests here should never rely on human inspection, hence, they shouldn't produce any permanent files.

If examples are the motive for the tests (as it is explicitly stated for one of them) it should be added to the examples/ folder and be part of the documentation.

Type of change:

• Bug fix.
• New feature.
• Documentation update.

Checklist:

This checklist can be used as a help for the reviewer.

• Is the code easy to read and understand?
• Are comments for humans to read, not computers to disregard?
• Does a new feature has an accompanying new test (in the CI or unit testing schemes)?
• Has the documentation been updated as necessary?
• Does this close the issue?
• Is the change limited to the issue?
• Are errors handled for all outcomes?
• Does the new feature provide new restrictions on dependencies, and if so is this documented?

Comments:

This partly closes the issue, since the tests should still be further tidied.

[francescalb]

A lot of changes have been made here, not only ensuring that produced files do not persist after running pytest. I think most of them are cosmetic, but can we talk through them so I can learn a little here?

[francescalb]

I suggest loading emmo with get_emmo()

[francescalb]

Good