title: digraph object has no attribute 'add_path'
body: <p>Scikit fuzzy rule formation cannot be done due to this error. Skfuzzy code and error message is shown below</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>import matplotlib.pyplot as pit
  import skfuzzy as fuz
  import numpy as np
  from skfuzzy import control as ctrl
  
  s=ctrl.Antecedent(np.arange(0,11,1),'service')
  q=ctrl.Antecedent(np.arange(0,11,1),'quality')
  t=ctrl.Antecedent(np.arange(0,26,1),'tip')
  
  s.automf(3)
  q.automf(3)
  
  t['low']=fuz.trimf(t.universe,[0,0,13])
  t['medium']=fuz.trimf(t.universe,[0,13,25])
  t['high']=fuz.trimf(t.universe,[13,25,25])
  
  
  rule1=ctrl.Rule(q['poor'] | s['poor'], t['low'])
  rule2=ctrl.Rule(q['poor'] | s['average'], t['medium'])
  rule3=ctrl.Rule(q['poor'] | s['good'], t['high'])
  
  tip_ctrl=ctrl.ControlSystem([rule1,rule2, rule3])
  tip=ctrl.ControlSystemSimulation(tip_ctrl)
  tip.input['service']=2.1
  tip.input['quality']=5.6
  tip.compute()
  print(tip.output['t'])
  #tip.view(sim=tip)
  #pit.show()</code></pre>
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  </p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>Traceback (most recent call last):
    File "/home/amavasi/.vscode/extensions/ms-python.python-2019.11.49689/pythonFiles/ptvsd_launcher.py", line 43, in &lt;module&gt;
      main(ptvsdArgs)
    File "/home/amavasi/.vscode/extensions/ms-python.python-2019.11.49689/pythonFiles/lib/python/old_ptvsd/ptvsd/__main__.py", line 432, in main
      run()
    File "/home/amavasi/.vscode/extensions/ms-python.python-2019.11.49689/pythonFiles/lib/python/old_ptvsd/ptvsd/__main__.py", line 316, in run_file
      runpy.run_path(target, run_name='__main__')
    File "/usr/lib/python3.7/runpy.py", line 263, in run_path
      pkg_name=pkg_name, script_name=fname)
    File "/usr/lib/python3.7/runpy.py", line 96, in _run_module_code
      mod_name, mod_spec, pkg_name, script_name)
    File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
      exec(code, run_globals)
    File "/home/amavasi/Public/ruthu/skfuzzy learn.py", line 22, in &lt;module&gt;
      tip_ctrl=ctrl.ControlSystem([rule1,rule2, rule3])
    File "/home/amavasi/.local/lib/python3.7/site-packages/skfuzzy/control/controlsystem.py", line 47, in __init__
      self.addrule(rule)
    File "/home/amavasi/.local/lib/python3.7/site-packages/skfuzzy/control/controlsystem.py", line 107, in addrule
      self.graph = nx.compose(self.graph, rule.graph)
    File "/home/amavasi/.local/lib/python3.7/site-packages/skfuzzy/control/rule.py", line 295, in graph
      graph.add_path([t, self])
  AttributeError: 'DiGraph' object has no attribute 'add_path'
  Terminated</code></pre>
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  <p>inside the rule.py module digraph object is created and add_path attribute is referenced from digraph object instead of networkx object. When I changed the reference to networkx object the error shows up saying add_path requires more attributes.</p>