Duffing system model generators for pulse simulator (Qiskit#516)

* Created new file for helper functions for creating HamiltonianModel and PulseSystemModel objects for duffing oscillator systems

* increased max shots in pulse_simulator

* added PulseSystemModel.control_channel_index() along with tests

example/pulse_sim.ipynb

@@ -56,9 +56,9 @@
      "name": "stderr",
      "output_type": "stream",
      "text": [
-      "/Users/dpuzzuoli/Documents/projects/qiskit-terra/qiskit/providers/models/pulsedefaults.py:166: UserWarning: `qubit_freq_est` and `meas_freq_est` now have units of Hertz(Hz) rather than gigahertz(GHz).\n",
+      "/Users/dpuzzuoli/anaconda3/envs/QiskitDev/lib/python3.7/site-packages/qiskit/providers/models/pulsedefaults.py:155: UserWarning: `qubit_freq_est` and `meas_freq_est` now have units of Hertz(Hz) rather than gigahertz(GHz).\n",
       "  warnings.warn('`qubit_freq_est` and `meas_freq_est` now have units of '\n",
-      "/Users/dpuzzuoli/Documents/projects/qiskit-terra/qiskit/providers/models/backendconfiguration.py:363: UserWarning: `dt` and `dtm` now have units of seconds(s) rather than nanoseconds(ns).\n",
+      "/Users/dpuzzuoli/anaconda3/envs/QiskitDev/lib/python3.7/site-packages/qiskit/providers/models/backendconfiguration.py:356: UserWarning: `dt` and `dtm` now have units of seconds(s) rather than nanoseconds(ns).\n",
       "  warnings.warn('`dt` and `dtm` now have units of seconds(s) rather '\n"
      ]
     }

qiskit/providers/aer/backends/pulse_simulator.py

@@ -41,11 +41,12 @@ class PulseSimulator(AerBackend):
         'coupling_map': None,
         'url': 'https://github.com/Qiskit/qiskit-aer',
         'simulator': True,
+        'meas_levels': [0, 1, 2],
         'local': True,
         'conditional': True,
         'open_pulse': True,
         'memory': False,
-        'max_shots': 50000,
+        'max_shots': 10**6,
         'description': 'A pulse-based Hamiltonian simulator',
         'gates': [],
         'basis_gates': []