Background MCC model: allow ionization by incident particles that are not electrons

[RemiLehe]

I looks like the MCC ionization models explicitly assume that the incident species is an electron: https://github.com/ECP-WarpX/WarpX/blob/development/Source/Particles/Collision/BackgroundMCC/ImpactIonization.H#L160 It would be good to generalize this.

[roelof-groenewald]

Here is a branch in which I started to work on having a different secondary electron species from the colliding species. This approach can be used here since it will similarly take 3 species as input for the ionization collisions. https://github.com/ModernElectron/WarpX/tree/mcc_diff_sec_elec_species