A few improvements

[tylerflex]

1. We could add more to the .gitignore. After running a few notebooks, the following got tracked.

	.ipynb_checkpoints/
	data/simulation.hdf5
	mode_solver.hdf5
	simulation_data.hdf5

2. When I black tidy3d, it seems to affect the files in here (any of the .py scripts). Is there a way to disable this? or maybe we just need to apply black to this repo.
3. speaking of black, I tried updating black and applying it to the front end, but some things were a bit weird. maybe after the holidays we can do this together.

might have more, just a few to start after playing around with it!