code: def get_heat_kernel(network_id):
      
      url = ndex_relevance + '/%s/generate_ndex_heat_kernel' % network_id
      res = ndex_client.send_request(url, {}, is_json=True, use_get=True)
      if res is None:
          logger.error('Could not get heat kernel for network %s.' % network_id)
          return None
  
      kernel_id = res.get('kernel_id')
      if kernel_id is None:
          logger.error('Could not get heat kernel for network %s.' % network_id)
          return None
      return kernel_id
docs: Return the identifier of a heat kernel calculated for a given network.
  
      Parameters
      ----------
      network_id : str
          The UUID of the network in NDEx.
  
      Returns
      -------
      kernel_id : str
          The identifier of the heat kernel calculated for the given network.