adding xacc observe example

Signed-off-by: Alex McCaskey <mccaskeyaj@ornl.gov>

tools/compiler/examples/deuteron_with_observation.cpp

+#include "xacc_runtime.hpp"
+#include <iostream>
+
+__qpu__  void ansatz(qbit &q, double t0) {
+  X(0);
+  Ry(t0, 1);
+  CX(1, 0);
+}
+
+int main(int argc, char **argv) {
+  xacc::Initialize(argc, argv);
+
+  auto hamiltonian = xacc::getObservable("pauli", "5.907 - 2.1433 X0X1 "
+                                                  "- 2.1433 Y0Y1"
+                                                  "+ .21829 Z0 - 6.125 Z1");
+
+  for (auto &t : xacc::linspace(-3.14, 3.14, 40)) {
+
+    // Allocate a register of 2 qubits
+    qbit q = xacc::qalloc(2);
+
+    #pragma xacc observe hamiltonian
+    ansatz(q, t);
+
+    auto energy = q->getInformation("observation");
+    std::cout << "E(" << t << ") = " << mpark::get<double>(energy) << "\n";
+  }
+
+  xacc::Finalize();
+
+  return 0;
+}
\ No newline at end of file