Add concentration-dependent transport coeffs

[Raymond Smith]

We should be able to solve Flux = -D(c)*grad(c) with non-constant D(c). While we're at it, we might as well make diffusivity functions as well for Flux = -D(c)*c*(1-c)*grad(mu).

These functions should go in the props_am.py file in a new class.

[Raymond Smith]

Merged in D-of-c (pull request #40)

Add D(c) functionality for solid. Resolves issue #32.

→ <<cset 7f3c5819c8a5>>