Performance tuning. I've refactored a bunch of code for the eval_parameters. is_changing was taking up way to much CPU time. is_changing should be called in on_build and the result cached. This happens now in Element._setup_changing_params. is_changing is now a Set(Parameters). Simulations.any_frequencies_changing is now an attribute to save that getting recalled again and again. Shaved 0.7s off of a 2s simulation.