BQM.subbqm or similar

Something similar to NetworkX's [Graph.subgraph](https://networkx.github.io/documentation/stable/reference/classes/generated/networkx.Graph.subgraph.html#networkx.Graph.subgraph) would be good. Would obviously need a BQMView object to implement that version though.

Note this is distinct from _induced_ subgraphs as in dwave-hybrid's [bqm_induced_by](https://docs.ocean.dwavesys.com/projects/hybrid/en/stable/reference/generated/hybrid.utils.bqm_induced_by.html#hybrid.utils.bqm_induced_by) function.