Allow any qubit type in quimb density matrix

[mpharrigan]

Used to restrict to linequbit just to make positioning easier when drawing the tensor networks. Now instead of using q.x, we use qubits.index(q) to find a qubits linear position.

[dstrain115]

Should we document that the qubit index determines the positioning somewhere?

Otherwise, it seems fine to me.

[mpharrigan]

added a line to the docstring

[CirqBot]

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