Add NEB [WIP]

[ElliottKasoar]

Resolves #375

Starting point for NEB.

It's a bit fiddly, as most calculations are designed to take in a single input structure, whereas this requires two.

Would it also be useful to allow the full image band as an input, in which case interpolation could be skipped?

To do:

• Add tests
• Fix images/bands, possibly via pymatgen's AseAtomsAdaptor, as used in the tutorial
• Add CLI
• Update documentation

Note: needs rebasing after #392

[alinelena]

for the neb, since interpolate is a form of magic, I suggest we implement 3 solutions..

[ ] ase way
[ ] pymatgen way
[ ] user way, user provides the images