Some Backwards Integrations Fail when using the C++ Backend.

[jrenaud90]

See the pytest.skip in the accuracy test in "test_a_pysolve_ivp.py" test file. Most (all?) of these are failing at the np.allclose check. I have manually checked some other cases with backward integration and it worked fine. Not sure what the trigger is.

This is likely to do with the backend's messy indexing when it comes to forward and backward integration. Code could really use a second pair of eyes to help clean it up and squash those bugs.

Will need to check that any index fixes still work for backward integration with t_eval and dense_output are set.