Add support for Freundlich isotherms as an InterfaceKernel

[ke7kto]

Reason

The Freundlich isotherm is a commonly accepted model representing sorptivity at an interface which empirical data has been fit to. This is necessary to model solid-vapor interface equilibrium. A henrian regime is also included for low concentration calculations, with a transition region determined by material properties.

Design

Add an interfacekernel, FreundlichPenaltyInterface, which accepts material properties relevant to the interface (these are properties relevant in the solid phase defaulting to As, Bs, Cs, Ds, Es, d1s, d2s). I have a current implementation, but it does not seem to work well with changes in temperature over time across the interface.

Impact

Will allow Freundlich isotherms to be modeled.