Muticontroller plot warning

[jarmarshall]

Noted during work on pull request #350 :

/Users/james/Google Drive/Home/Git/MuMoT/mumot/views.py:550: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes
currently reuses the earlier instance.  In a future version, a new instance will always be
created and returned.  Meanwhile, this warning can be suppressed, and the future behavior
ensured, by passing a unique label to each axes instance.
  plt.subplot(self._numRows, self._numColumns, subPlotNum)

[jarmarshall]

This issue is specific to nested multicontrollers - unfortunately following the solution proposed by the warning breaks multicontroller nesting, and puts all component plots at the same level. For now a workaround is to ignore the warning, but in the future this issue will need to be fixed properly, when matplotlib makes good on its threat...