code: def power(self):
          r
          if self.scale_by_freq == False:
              return sum(self.psd) * len(self.psd)
          else:
              return sum(self.psd) * self.df/(2.*numpy.pi)
docs: r"""Return the power contained in the PSD
  
          if scale_by_freq is False, the power is:
  
          .. math:: P = N \sum_{k=1}^{N} P_{xx}(k)
  
          else, it is
  
          .. math:: P =  \sum_{k=1}^{N} P_{xx}(k) \frac{df}{2\pi}
  
          .. todo:: check these equations