Concentration-dependent diffusion coefficient

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).