Add function for 2-qubit tomography

[manul-pointer]

two_square_density_matrix() gives the density matrix for any pair of squares.

This requires running 9 variants of the circuit with different gates at the end tweaking the two qubits of interest.

Error correction and noise mitigation are disabled, since the qubits undergoing tomography can't be measured in the computational basis.

The density matrix can be viewed from interactive_board.py with the command, e.g., density f3 f5.

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Before merging I'd want to remove some of the duplicated code between the new function and sample_with_ancilla.