[WIP] Add automated test for ion impact ionization

[RemiLehe]

TODO: update analysis script and recreate checksums

[EZoni]

Just a side comment, as far as creating the checksum file is concerned, I think this could be a good opportunity to verify that the steps in our how-to documentation (https://warpx.readthedocs.io/en/latest/developers/how_to_test.html#how-to-add-automated-tests) work properly, in particular step 7. below (I'm not sure if developers actually do this, instead of copying existing checksums and then overwriting them):

[roelof-groenewald]

@EZoni I was able to reset the benchmarks using the instructions you linked 🎉 .
I guess we'll see if the checksum values from my local system matches what the Azure systems get.

@RemiLehe I modified the scattering logic to avoid the nan values from before (at least for the test you created). Could you please check if the update works for your actual use case?

Btw, I checked that the electron impact ionization test still match theory (as close as it did before), and it looks good:

[roelof-groenewald]

TODO: update analysis script and recreate checksums

@RemiLehe are you planning on updating the analysis script to get the theoretical ion impact ionization rate and the effect on the ion temperature?

[roelof-groenewald]

I noticed the test runs on 2 procs but with max_grid_size=8 there was only 1 tile, so parallel execution was not really tested.