Fix opencv segmentation fault

[aliberts]

What this does

• Adds lock mechanism in OpenCVCamera for thread safe operation
• Adds yield in pytest monkeypatching fixture for cameras (actually doesn't work)
• Removes inheritance of MockVideoCapture from cv2.VideoCapture, which was causing ugly errors when trying to monkeypatch cv2.VideoCapture with it.
• Improve MockVideoCapture for more accurate behavior
• Use np.testing.assert_allclose instead of assert np.allclose for displaying errors nicely

TODO (this PR or another one):

• Will probably need to add the same logic to motors and realsense

How it was tested

Running pytest -v -k "opencv" tests/test_cameras.py doesn't raise any segmentation fault anymore, but does still raise assert errors on np.allclose (separate issue)

EDIT: For mock=True this is now fixed. It was caused by the monkeypatch not actually being applied and therefore was using the real camera instead of the mock one.

How to checkout & try? (for the reviewer)

pytest -v -k "opencv" tests/test_cameras.py

[Cadene]

Thanks!