Implement concentration dependent pore and surface diffusion

[lieres]

[sleweke]

Partially implemented (surface diffusion, 1D general rate model) in 444597c5231d660e0207d5f9f2f18adbbeac6e02.

[schmoelder]

Hey guys, since we're currently in the process of cleaning up some old issues, could you please comment on what's already implemented, how to use it and what's still missing? Thanks!