Fixes #234: Add `brute_force` method to the QAOA Workflow class

[lumapools]

Description

We can now call QAOA.brute_force() to brute force the ground state of its cost Hamiltonian.

• [x] Previous and new tests pass locally

Description

• Fixes #234
• Added to the QAOA Workflow class, a new method brute_force() that indirectly computes the ground state energy of the QAOA Hamiltonian.
• Kept the bounded default parameter from the ground_state_hamiltonian method, hence the original 25 qubit limit

Checklist

• [x] I have performed a self-review of my code.
• [x] I have commented my code and used numpy-style docstrings
• [x] I have made corresponding updates to the documentation.
• [x] My changes generate no new warnings
• [x] I have added/updated tests to make sure bugfix/feature works.
• [x] New and existing unit tests pass locally with my changes.

Type of change

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Added a test in test_workflows.py called test_brute_force which checks that:

• The result of the computed ground state still matches the one from ground_state_hamiltonian
• An uncompiled QAOA's test_brute_force raises an error
• The bounded parameter still acts as it used to with utilities.ground_state_hamiltonian and raises an error when necessary

[vishal-ph]

@lumapools Thank you for your attempt implemeting issue #234. Please give us a few days while we review the code!

[Q-lds]

@lumapools as agreed I have merged the fixes from @GiacomoFrn. Please, go ahead and update your tests and then we are good to go :)

[lumapools]

@Q-lds Closed this PR due to some synchronization issues and not being able to fetch the latest changes. I made a fresh clone, and the new PR is #246 where only the tests appear.