Add NEB convergence and restart

[ElliottKasoar]

As requested by several of the contributors to ML-PEG, this:

1. Stores the bool returned by the optimisation step, which could be used for 'convergence'
2. Allows and tests restarting/continuing NEBs e.g. to tighten criteria, or to change methods e.g. https://github.com/jungsdao/ml-peg/blob/3ea9d6a02f06ac0187bc22f324b737618fa8dfbd/ml_peg/calcs/nebs/surface_reaction/calc_surface_reaction.py

This also adds to the tutorial to discuss these, and the limitations of the converged flag.