Added I/O file for voronoi (same as octree for now)
Added grid type for Voronoi grid
Added cell type for voronoi
Added KDTree routines
Working implementation of Voronoi mesh
Added the initial file for the Voronoi grid helper code.
Updated to use voronoi_helpers
Remove unused file
Ensure huge assumes correct type in kdtree
Move points to global scope to avoid pointer getting corrupt
Fix bug for cells with neighbors that are cell 0 in Python notation. Also fix intersection with boundaries.
Ensure that random_position_cell cannot stay stuck in an infinite loop
Changes following the code review in #1.

- Fix code style,
- use Numpy arrays and related API more thoroughly and consistently,
- made the neighbours table a cached property,
- change the way bounding boxes are presented in the table.
An optimization in the computation of the simplex's volume, some minor refactoring and naming fixes.
Implement the calculation of the volume of a 3d simplex in C for performance, and revert to assigning a volume of -1 to protruding cells.
Compute the neighbours list directly as a numpy array and start using the multiprocessing utils.
Rewrite more functions in C and parallelize the computation of volumes and bbs.
Minor cleanups and commenting.
Run autopep8 on voronoi_helpers.py.
Fix spelling errors in comment.