filter: Add FilterSphere

[ctbur]

Changes

Add the FilterSphere filter than can filter points for a sphere given a center and a radius.

Unfortunately I had to use a scalar converter that needs to be hand-picked and match the multidimensional converter used in the tree. Please let me know how it can be improved.

Verification

Added a test case.

[improbable-til]

The normal converters can only convert PhPoint instances. I think it would be good if you could use the same converter that is used for the PhTree itself. I see two options:

1. Extend one of the existing converters and add pre()/post() functions for scalars. This could then be used to also create the PhTree itself.
2. Adapt the filter to work with PhPoints.
   • Convert the sphere center to an internal point
   • Use < / > to identify/create the closest corner point of the node to the sphere center. The result is an internal point. (It is safe to compare internal scalars, but it is not safe to add/substract/multiply/... them).
   • Convert the internal corner point to an external point to calculate the distance to the sphere center.

[ctbur]

Thanks. Approach 2. worked and gave a much cleaner result. We actually just need to clamp the center point in the node's AABB to get the closest distance, so only comparisons are needed, no calculations.

[improbable-til]

LGTM.
Nice contribution, thanks!