intensity estimates for new ripley functions are very incorrect sometimes

[ljwolf]

I noticed this problem when writing tests for the new numpy-based performant Ripley statistics. When using a shapely polygon's convex hull, I was getting basically the correct intensity, but when I was using scipy, I was getting extreme overestimates of the intensity.

I narrowed it down to an issue with how scipy.spatial.ConvexHull.area works scipy/scipy#12290.

Depending on the fix there we need to either:

1. move to using hull.volume when getting the area from a scipy convex hull if upstream decides to adopt docstring clarifications
2. keep using area if upstream decides to adopt the proposed changes for area/volume of 2-d shapes.

[ljwolf]

I moved to using volume until I hear from scipy.