Add power function kernel to Urban Nature Access

[phargogh]

In the Urban Nature Access design doc, a power function is defined as one of the possible decay functions:

$$ f(d_{ij}, d_0) = d_{ij}^{(-\beta)} $$

Where the function is defined for all $d_{ij} \leq d_0$

While the kernel is trivial to define, $\beta$ is not defined anywhere in the document. Because of this lack of specification, @dcdenu4 and I decided to pull the power function from the initial release of the model. This will allow us to more carefully get consensus on the implications of adding the function.

Tasks:

1. Understand how this was intended to be defined (e.g. define $\beta$ anywhere a search radius is defined)
2. Implement in the model and the workbench (Urban Nature Access - Split radii by population groups #1150 includes commented-out bits with the partial implementation)
3. Add example to the sample data
4. PR in to InVEST

This is related to #722 , but extracted into its own issue.

Estimated time remaining

1-3 solid days, broken up a bit while getting input from science leads.