Do we need test_trapping_cubic_library()?

[Jacob-Stevens-Haas]

AFAIK, we don't do trapping with a cubic library. The current test just verifies that it's possible to fit a SINDy model with a trapping optimizer and a custom library with the following functions?

    library_functions = [
        lambda x: x,
        lambda x, y: x * y,
        lambda x: x**2,
        lambda x, y, z: x * y * z,
        lambda x, y: x**2 * y,
        lambda x: x**3,
    ]