Add voronoi visualization if compute voronoi/atom is used

[andeplane]

I tried it and it worked. We need to compile voro++. LAMMPS needs to have symlinks in /path/to/lammps/lib/voronoi pointing to the voro++/src/ folder (or just compile with the python script provided in there). Atomify needs to include the voro++/src/ folder and link with -lvoro++.

The output from LAMMPS is then 2 or 3 outputs (as explained here which can be used to color atoms.

[andeplane]

We don't have access to the actual cells though, so we cannot render the voronoi cells without doing more calls to voro++ (which we probably should do from atomify).

[andeplane]

Or probably add another voronoi step only in atomify so we can look at voronoi cells of subsets (after removing atoms etc)?

[andeplane]

Since voronoi is heavy, the user should ask for it at a paused timestep.

[andeplane]

We can ship atomify with voro++ so that atoms get colored, but we should wait with more advanced stuff.