BUG: Correct convergence conditions in function iteration

[oyamad]

See #171

This pattern, which is found in several files,

    while i < max_iter and error > tol:

    ...

    if i == max_iter:
        print("Failed to converge!")

    if verbose and i < max_iter:
        print(f"\nConverged in {i} iterations.")

should be corrected to, e.g.,

    if error > tol:
        print("Failed to converge!")
    elif verbose:
        print(f"\nConverged in {i} iterations.")

[mmcky]

thanks @oyamad.

@shlff do you have time to review this issue and put together a PR?