Unit Testing in Docker

[dhblum]

This issue is to catch an error thrown if running unit tests within a Docker container with no access to a display.

Traceback (most recent call last):
  File "/usr/local/git/mpcpy/MPCPy/unittests/test_optimization.py", line 80, in test_optimize
    self.check_df(df_test, 'simulate_optimal_default.csv');
  File "/usr/local/git/mpcpy/MPCPy/unittests/testing.py", line 103, in check_df
    self._check_values(df_test, df_ref)
  File "/usr/local/git/mpcpy/MPCPy/unittests/testing.py", line 231, in _check_values
    plt.figure()
  File "/usr/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 527, in figure
    **kwargs)
  File "/usr/lib/python2.7/dist-packages/matplotlib/backends/backend_tkagg.py", line 84, in new_figure_manager
    return new_figure_manager_given_figure(num, figure)
  File "/usr/lib/python2.7/dist-packages/matplotlib/backends/backend_tkagg.py", line 92, in new_figure_manager_given_figure
    window = Tk.Tk()
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1818, in __init__
    self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
TclError: couldn't connect to display ":0"