Voronoi mesh projections

[matthewturk]

The routine pixelize_sph_kernel_projection works fine for SPH kernels. However, for voronoi tesselations, this does not produce ideal results. Gridding and slicing vornoi tesselations is not as tricky, as there we can set number of neighbors to 1 and use a unitary-weighted smoothing kernel. But for projections, it's harder.

Here is an example of using the sph kernel for the arepo bullet cluster simulation when projecting density.