Concentration-dependent diffusion coefficient

[truecruiseraurora]

I would like to be able to specify the diffusion coefficient as a function of concentration. This could be a parameter in a material class, like thermal_cond or heat_capacity, for example:
D_conc=lambda c_m: 3 * c_m + 2,
This would be a function that is then multiplied by the Arrhenius function (with E_0 and E_D parameters).

[RemDelaporteMathurin]

Thanks for opening this issue. If we want to remain general then the diffusion coefficient should be able to temperature-dependent, concentration dependent, space-dependent, time-dependent.

@jhdark can we use the Value class for this? I don't think it can support species depdence. an example usage would be

D = F.Value(lambda T, c1: 1 + T + c1, species_dependent_value={"c1": species1})

mat = F.Material(..., D=D)