Capability for plasma loading BC

[simopier]

Reason

A plasma loading BC could be useful to define the surface concentration for a given particle flux and diffusivity. This is used a lot in the literature, see this paper.

Design

Create a BC that can handle plasma loading BC, or provide documentation for user on how to set this up with existing capabilities.

Impact

Enhance capabilities for plasma loading.

Suggested by @stephen-dixon

[RemDelaporteMathurin]

This expression will be different if you consider 1) instantaneous recombination (Kr = infinity) 2) a negligible partial pressure of hydrogen (no dissociation)

See https://github.com/RemDelaporteMathurin/FESTIM/pull/557