Added the cpp version of trotter test case

#include <chrono>

// Trotter evolution

__qpu__ void trotter_evolve(qreg q,

                            std::vector<std::shared_ptr<Observable>> &exp_args,

                            int n_steps) {

  for (int i = 0; i < n_steps; ++i) {

    for (auto &exp_arg : exp_args) {

      exp_i_theta(q, 1.0, exp_arg);

    }

  }

}

int main() {

  // Helper to create general Heisenberg Hamiltonian

  const auto heisenberg_ham = [](int nbQubits) {

    const double Jz = 1.0;

    const double h = 1.0;

    auto H = -h * X(0);

    for (int i = 1; i < nbQubits; ++i) {

      H = H - h * X(i);

    }

    for (int i = 0; i < nbQubits - 1; ++i) {

      H = H - Jz * (Z(i) * Z(i + 1));

    }

    return H;

  };

  const std::vector<int> n_qubits{5, 10, 20, 50, 100};

  const int nbSteps = 100;

  for (const auto &nbQubits : n_qubits) {

    auto ham_op = heisenberg_ham(nbQubits);

    auto op_terms = ham_op.getNonIdentitySubTerms();

    auto q = qalloc(nbQubits);

    auto start = std::chrono::high_resolution_clock::now();

    auto nInsts = trotter_evolve::n_instructions(q, op_terms, nbSteps);

    auto stop = std::chrono::high_resolution_clock::now();

    auto duration =

        std::chrono::duration_cast<std::chrono::microseconds>(stop - start);

    const double elapsed_secs = (double)duration.count() / 1e6;

    print("n_qubits =", nbQubits, "; n instructions =", nInsts,

          "; Kernel eval time:", elapsed_secs, " [secs]");

  }

}