change directory name for `zexperimental`

[amal-ghamdi]

General description of task

Change directory name for tests from zexperimental to better name and ensure test_mcmc.py still executed last.

Current status we have:

zexperimental/test_mcmc.py
zexperimental/test_recommender.py
zexperimental/test_algebra.py
zexperimental/test_productgeoemtry.py

After this PR, we change zexperimental to experimental and move all test files in experimental to the top level of tests but keeping only test_recommender.py in experimental because those features are no longer experimental except the recommender because it is still an experimental module. To ensure test_mcmc.py is still the last test files to be executed, we changed its name from test_mcmc.py to test_z_mcmc.py to make sure it is the last file to be executed (this is due to some issue with numpy randomness).

Definition of Done (Feature/change)

• change directory name (see the above description).

Definition of Done (Mandatory)

• (N/A) Documentation added (docstrings on all public methods/classes)
• (N/A) Unit tests added/updated (and passing!)
• Code reviewed and approved by 2 CUQI-maintainers
• (N/A) Online documentation is rendered correctly (esp. math)

Definition of Done (Optional)

• (N/A) HowTo/tutorial added

[jakobsj]

DoD approved - thanks for your patience :-)