Add "power law" blending

[otto-link]

Blend two (or more) fields using a power law averaging:

$$f_\text{blend} = \frac{\sum_i f_i^{\alpha+1}}{\sum_i f_i^\alpha}$$

Blending function whose behavior varies with the bias $\alpha$ from a simple average of the heights at each node when $\alpha=0$, approaching $\max(f_i)$ (the maximum height) as $\alpha$ approaches $\infty$.