title: AttributeError related to multiprocessing when using extract_features from tsfresh
body: <p>I have a data frame which contains multivariate time series data. It has columns: fundId (each entity), periodId(time stamp), v1,  v2, v3 ...</p>
  
  <p>I want to extract features from those times series for each fund over time. Like: fundId v1.max, v1.min, v1.median, ..., v2.max, v2.min, v2.median, ...</p>
  
  <p>I encountered an AttributeError when use function extract_features from library tsfresh. I'm new to Python but I think it might related to multiprocessing.</p>
  
  <p>The trackback shows:</p>
  
  <p>Traceback (most recent call last):
    File "mypath\lib\site-packages\IPython\core\interactiveshell.py", line 3325, in run_code
      exec(code_obj, self.user_global_ns, self.user_ns)
    File "", line 1, in 
      HFeatures = extract_features(FundCharc, column_id='fundId', column_sort='periodId')</p>
  
  <p>File "mypath\lib\site-packages\tsfresh\feature_extraction\extraction.py", line 178, in extract_features
      distributor=distributor)</p>
  
  <p>File "mypath\lib\site-packages\tsfresh\feature_extraction\extraction.py", line 313, in _do_extraction
      progressbar_title="Feature Extraction")</p>
  
  <p>File "mypath\lib\site-packages\tsfresh\utilities\distribution.py", line 349, in <strong>init</strong>
      self.pool = Pool(processes=n_workers)</p>
  
  <p>File "C:\ProgramData\Miniconda3\lib\multiprocessing\context.py", line 119, in Pool
      context=self.get_context())</p>
  
  <p>File "C:\ProgramData\Miniconda3\lib\multiprocessing\pool.py", line 176, in <strong>init</strong>
      self._repopulate_pool()</p>
  
  <p>File "C:\ProgramData\Miniconda3\lib\multiprocessing\pool.py", line 241, in _repopulate_pool
      w.start()</p>
  
  <p>File "C:\ProgramData\Miniconda3\lib\multiprocessing\process.py", line 112, in start
      self._popen = self._Popen(self)</p>
  
  <p>File "C:\ProgramData\Miniconda3\lib\multiprocessing\context.py", line 322, in _Popen
      return Popen(process_obj)</p>
  
  <p>File "C:\ProgramData\Miniconda3\lib\multiprocessing\popen_spawn_win32.py", line 33, in <strong>init</strong>
      prep_data = spawn.get_preparation_data(process_obj._name)</p>
  
  <p>File "C:\ProgramData\Miniconda3\lib\multiprocessing\spawn.py", line 172, in get_preparation_data
      main_mod_name = getattr(main_module.<strong>spec</strong>, "name", None)</p>
  
  <p>AttributeError: module '<strong>main</strong>' has no attribute '<strong>spec</strong>'</p>
  
  <pre class="lang-py prettyprint-override"><code>from tsfresh import extract_features
  HFeatures = extract_features(FundCharc, column_id='fundId', column_sort='periodId')
  </code></pre>