Grid class should integrate with VolumeData classes of pymatgen

[jacksund]

pymatgen has BaseVolumetricData, VolumetricData, VoxelData, etc... classes that help with this. The badelf class for grid should integrate with these. By doing so, this app can more easily extend to programs besides VASP

[SWeav02]

Yeah I need to look into these more. My initial reasoning was that for very large grids the Elfcar and Chgcar classes take an unreasonable amount of time to read/write the files. But I haven't messed around with the more base classes you mentioned here and most likely they don't suffer from this issue.

I think the ideal situation is that one of these has methods that allow for reading/writing grids between different file types without to much going on in the background. I could just inherit from that and add the more specialized methods I need throughout the BadELF alg.

As an aside I've found the Grid class super useful when testing different aspects of BadELF so improving it is pretty high on my priority list. It just makes life super easy.