Add snow damping effect for Dirichlet temperature forcings

Also renames TempGradTransform type(s) to TemperatureGradientEffect for more clarity.

TemperatureGradient accepts an implementation of TemperatureGradientEffect which can be used to define specialized dispatches that apply an effect to the temperature forcing. For the SnowDamping effect, this is based on the additional Dsn (snow depth) forcing.