Guidance on adding secondary electron emission

[budjensen]

I would like to add secondary electron emission to WarpX. Could you point me in the best way to get started on this?

I would like to build the feature so that in the future that we can incorporate both energy and angular dependence to the secondary emission coefficient, but for speed I'd prefer to implement a basic version which accepts a coefficient, $\gamma_\mathrm{SEE}$, for each species.

SEE emission would be determined by drawing a random number $R\in[0,1]$ and adding a secondary electron if $R>\gamma_\mathrm{SEE}$ (If $\gamma_\mathrm{SEE} > 1$, floor($\gamma_\mathrm{SEE})$ particles would be added before comparing R and $\gamma_\mathrm{SEE}$). All SEE electrons would have a pre-specified energy and be isotropically scattered in velocity.

[budjensen]

I've opened this as a draft PR! See #5715 for continuing work