Research: ExB Instabilities

[Yin-YinjianZhao]

This PR is used for code comparison and should never be merged.
Branch ExB is the modified version, and ExB_old_warpx is the original version of WarpX before modification.

This PR is for simulating a test presented in the paper:
"2D radial-azimuthal particle-in-cell benchmark forE×B discharges", Plasma Sources Sci. Technol.30(2021) 075002 (15pp), DOI: 10.1088/1361-6595/ac0a4a.

Two additional features are needed that this version of WarpX does not have.

1. Virtual spatial direction motion and reinjection of particles. This feature is directly added by modifying the UpdatePosition function in WarpX.
2. Ionization injection according to a specific distribution function. This feature needs to add new particles and is done through PICMI callback functions.

• In the Bin folder, picmi.py is the base script to run the simulation.
• picmi_all_callbacks.py is an old version using all callback functions but very slow, and is only stored here as a reference (cannot be correctly ran).
• read.py is used to make plots.