Fix pioneer tutorial bug (define omega_data) (#379)

Author: jtgrasb

examples/tutorial_4_Pioneer.ipynb

@@ -306,6 +306,7 @@
     "datafile = 'data/pioneer_empirical_data.nc'\n",
     "empirical_data = xr.load_dataset(datafile)\n",
     "\n",
+    "omega_data = empirical_data.omega\n",
     "exc_coeff_data = empirical_data.exc_coeff_data_real + 1j*empirical_data.exc_coeff_data_imag\n",
     "Zi_data = empirical_data.Zi_data_real + 1j*empirical_data.Zi_data_imag\n",
     "Zi_stiffness = empirical_data.Zi_stiffness\n",