add subgraph/"supergraph" functions to AtomGraph

[rkurchin]

It would be really cool to be able to filter based on feature values (for example, if the contextual features idea from this issue gets implemented, then we could for example take only the adsorbate atoms or only the surface atoms.

Would need to make sure to modify feature matrix and element list appropriately and also recompute laplacian, but this should be easy.

Similarly, would be cool to construct larger graphs (e.g. adsorbed species on a surface) by just inputting two smaller ones and specifying which edges need to be added (e.g. an edge from node X in graph 1 to node y in graph 2), though how to handle working out weights would need to be figured out...