Added first Heisenberg model example

#include "qcor_qsim.hpp"

// High-level usage of Model Builder for Heisenberg model.

// The Heisenberg Hamiltonian is parameterized using ArQTiC scheme.

// Compile and run with:

/// $ qcor -qpu qpp TdWorkflowHeisenbergModel.cpp

/// $ ./a.out

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

  using ModelType = qcor::qsim::ModelBuilder::ModelType;

  // Example ArQTiC input:

  // *Jz 

  // 0.01183898

  // *h_ext

  // 0.01183898

  // *initial_spins 

  // 0 0 0

  // *freq

  // 0.0048

  // *ext_dir

  // X

  // *num_spins

  // 3

  auto problemModel = qsim::ModelBuilder::createModel(ModelType::Heisenberg,

                                                      {{"Jz", 0.01183898},

                                                       {"h_ext", 0.01183898},

                                                       {"freq", 0.0048},

                                                       {"ext_dir", "X"},

                                                       {"num_spins", 3}});

  // Workflow parameters:

  // *delta_t

  // 3

  // *steps

  // 20

  auto workflow = qsim::getWorkflow(

      "td-evolution", {{"method", "trotter"}, {"dt", 3.0}, {"steps", 20}});

  // Result should contain the observable expectation value along Trotter steps.

  auto result = workflow->execute(problemModel);

  // Get the observable values (average magnetization)

  const auto obsVals = result.get<std::vector<double>>("exp-vals");

  // Print out for debugging:

  for (const auto &val : obsVals) {

    std::cout << "<Magnetization> = " << val << "\n";

  }

  return 0;

}