Add IPOPT Optimizer

[schmoelder]

This PR adds support for interior point optimzer IPOPT using cyipopt.

@flo-schu I guess this still needs some work but it's been in a local branch for a long time and maybe it would be a good opportunity to include it in the test runners and maybe even for the optimization study.

Could you please jog my memory and briefly tell me what is required to run the test_optimizer_behaviour using pytest?

[flo-schu]

The new optimizer needs to be inserted here:

@pytest.fixture(params=[
    SLSQP,
    TrustConstr,
    U_NSGA3,
    GPEI,
    NEHVI,
])
def optimizer(request):
    return request.param()

that's it. Then call the test with pytest tests/test_optimizer_behavior.py::test_from_initial_values

By the way, in dev currently both, test_from_initial_values and test_convergence are used. Both tests are almost the same and I don't think the test_convergence test is necessary.

[schmoelder]

@hannahlanzrath, this is another optimizer that uses gradient information. This is WIP, but maybe you'd be up for testing it a bit, especially when considering how Jacobians (and Hesse) matrices computed?