[WIP] Add NEB, ApproxNEB jobs / workflows

[esoteric-ephemera]

Adding NEB workflows and schemas for VASP / ASE / MLFF. This hopefully brings atomate2 to full parity with the feature set of atomate

@hmlli is leading the port of ApproxNEB flows for VASP

Very open to suggestions, particularly for document schema structure

Key updates:

• Generic NEB schemas (one for single-barrier/hop NEB, one for combined pathways)
• Different VASP run logic / makers / validators for NEB jobs
• Parsing of VASP output handled by new NebTaskDoc class in emmet

To-do:

• Better parsing of NEB outputs (collated into container doc with analysis of barrier)
• ASE NEB optimizer
• Subclass ASE NEB for MLFF
• Tests