Add capability for Voronoi calculation to have a potential as an input

[JPchico]

If one wants to for example increase the lmax, one needs to re-run voronoi with a larger lmax, however, one can reuse an older potential which is already better converged. One would need to check that this potential matches the input parameters, and then make sure that the parser gets the name from this one instead of the one produced by Voronoi when gathering the data.