docs: def averageValues(self):
          """
          return the averaged values in the grid
          """
          assert self.opts['record_density'] and self.opts['method'] == 'sum'
          # dont increase value of partly filled cells (density 0..1):
          filled = self.density > 1
          v = self.values.copy()
          v[filled] /= self.density[filled]
  
          # ONLY AS OPTION??:
          v[~filled] *= self.density[~filled]
          return v