Remove `is_adjacent` from line and grid qubits.

LineQubit models a very sparsely connected topology. GridQubit describes devices with very dense connectivity. In a lot of real quantum hardware qubit connections are denser or more elaborate than simple lines, but still less dense than fully connected 2-D grids, see e.g. IBM Q device topologies below or Rigetti’s Aspen topology.

This issue proposes a new type of qubit class with behavior intermediate between LineQubit and GridQubit. In particular, the is_adjacent methods should be flexible enough to adhere to any topology specified at construction.

[maffoo]

+1 from me on removing is_adjacent on qubits. Adjacency is not a property of qubits, but a property of the devices they are embedded in (even if that device is the “unconstrained” device with an infinite lattice). The API should make this clear.

[Strilanc]

I don’t think this concept lands in a nice conceptual valley. It strikes me as very general (you can do any connectivity!) mixed with very specific (it has to be on a plane at integer coordinates).

I think we should wait until we actually need a type of qubit before defining it. And my instinct is to define specific qubit types (“OctagonalTilingQubit”) rather than to create a qubit so general that it can represent anything.