Start Python bindings for Raycast

This is only beginning of the bindings here. It's starting to get more
complicated due to the use of callbacks. To be able to define a callback
function in Python which is then called by a C++ algorithm requires quite a
bit of wrapping boilerplate.

The class PyCallback here will wrap a Python callable and call it with the
density value of the voxel. It's not very generic and at present it can't
pass the sampler itself since it's not available in the Python bindings.

Regardless, the new test added here (TestRaycast.py) works as expected and
hopefully we will be able to build up from here.

library/bindings/PolyVoxCore.i

@@ -75,3 +75,4 @@ EXTRACTOR(shortname, LargeVolume)
 %include "SurfaceMesh.i"
 %include "MarchingCubesSurfaceExtractor.i"
 //%include "CubicSurfaceExtractor.i"
+%include "Raycast.i"