Add support for nequip calculations

[alinelena]

No description provided.

[ElliottKasoar]

Did you say there are trained models we can use to test this?

From the calculator interface, it doesn't look like there's a default that can be automatically loaded like we have for the existing MLIPs.

[alinelena]

yes found some. https://pubs.aip.org/aip/jcp/article/160/15/154106/3282872/Global-machine-learning-potentials-for-molecular

[ElliottKasoar]

Great, thanks!

We should also update the README list of supported MLIPs to include it and ALIGNN, maybe in the same list but with an (experimental) label.

This should also be a good test of the dev tutorial, particularly as it's now a bit more complicated than when I originally added ALIGNN.