Add a new pickVoxel function.

Hopefully this pickVoxel function will fulfil most most need with
raycasting which aren't covered by the existing raycast functions.

It's essentially a wrapper around the current functions, using a custom
functor to store the position of a solid voxel as well as the previous
empty voxel. It doesn't require the user to provide a functor of their own,
just the value that they've assigned to empty voxels.

As well as the function itself, we also have a unit test as well as it
being built in the bindings.

library/bindings/Picking.i

@@ -0,0 +1,8 @@
+%module Picking
+%{
+#include "Picking.h"
+%}
+
+%include "Picking.h"
+
+EXTRACTORS(pickVoxel)

library/bindings/PolyVoxCore.i

@@ -90,3 +90,4 @@ EXTRACTOR(shortname, LargeVolume)
 %include "CubicSurfaceExtractor.i"
 %include "CubicSurfaceExtractorWithNormals.i"
 %include "Raycast.i"
+%include "Picking.i"