allow skipping derivative computations

[aplavin]

I have some usecases where derivative is almost never useful for range enclosure, but takes the majority of calculations.

This PR makes it possible to pass df = nothing and skip all derivative-related computations.

[schillic]

Thanks, this makes sense. May I ask you to add two changes?

1. Add a test for df=nothing.
2. Update the docstring of BranchAndBoundEnclosure (in algorithms.jl) to explain that df=nothing is a valid input that skips the check.

[aplavin]

done

[schillic]

The test failed. Please check again.