Improve and generalize boundary damping scheme

Replaces the previous "snow damping" temperature gradient effect with a general Damping effect. Also replaces TemperatureGradientEffect with a more general base type BoundaryEffect in the Boundaries module.

Damping takes a depth/thickness function of time as a forcing as well as a bulk thermal conductivity, and then provides a first order boundary condition by computing the gradient k(u_sub - u_top)/d(t) where d is the half distance of the upper-most grid cell + the damping depth at time t.

This can be used to apply simple, approximate damping effects from snow, water, vegetation, etc, so long as there is a thickness forcing available.