Support for atom forceSets

Since it's possible to have multiple 3d sets (e.g., a trajectory or optimization) it would be useful to have atomic forces as a `forceSets` attribute, e.g.

```
"forceSets" : [
[ numeric array of 3*N forces ],
[ numeric array of 3*N forces ],
…
]
```

This would be useful to monitor forces / gradients during an optimization or trajectory.