Upgrade SDE Euler solver to Milstein

[mountaindust]

See note in the docstring, and Milstein Method on Wikipedia. For now, we will assume that \sigma'(x) is zero for most applications, making this method equivalent to the current one.

[mountaindust]

Needed for porous media models. It's worth noting in the documentation somewhere that D for porous media should be the effective diffusion constant divided by volumetric water content. See this paper.