option to control Trotter steps on GHZ state

[babbush]

Many of our Trotter steps are naturally implemented on a linear array. However, many NISQ devices are planar. If one wants to control these linear Trotter steps on an ancilla using one of these planar arrays the natural thing to do is to create a GHZ state in a line of qubits above the linear system register. This GHZ state is basically one very long qubit. Then, each rotation in the Trotter step can be controlled on the qubit that is part of the GHZ immediately above it. I'd like to play around with such circuits.

However, right now we can only specify a single ancilla as the control qubit. Let's think about how we can change this to support the use case I described above.