Add tool for fractal dimension

[scw]

Fractal dimension is a frequently used tool for users who are generating covariates for the purposes of habitat prediction and modeling. A one-pass version can be computed as:

Where D is the output value, and S is slope in degrees.

This problem is best solved using FFTs, which aren't directly available, but we could do this with numpy arrays. There's a nice write-up of this approach on GIS.SE.

Pittman has also used FocalD for this.