Fixed errors in edge-cases and improved docs

- K-d tree builder now handles simplices embedded in split (hyper)planes
correctly
- Ray tracer now correctly handles the case where a primitive beyond a split
plane overlaps a primitive included in a node before the split plane
- CompositeScene now uses a bounding box to compute a correct near constraint
needed by the k-d tree traversal algorithm
- Finished the Basic Usage documentation
- Updated API documentation
- Added build_composite_scene function
- Made small changes for clang compatibility
- A few small bug-fixes
- Worked on better SIMD support for comparison operators