Update QAOA placement example (use aer with connectivity)

/// qaoa_placement.cpp: Topology placement example

/// qcor -qpu ibm:ibmq_paris  qaoa_placement.cpp -print-final-submission

/// Using the default placement:

/// qcor -qpu aer:ibmq_paris  qaoa_placement.cpp -print-final-submission

/// Change the placement:

/// qcor -qpu aer:ibmq_paris -placement enfield qaoa_placement.cpp -print-final-submission

__qpu__ void qaoa_maxcut(qreg q, std::vector<double> gamma,

                  std::vector<double> beta,

                  std::vector<std::pair<int, int>> graph_edges) {

      exp_i_theta(q, beta[step], ref_ham_term);

    }

  }

  Measure(q);

}

int main(int argc, char **argv) {