code: def steady_state_potential(xdata,HistBins=100):
        
      import numpy as _np
      
      pops=_np.histogram(xdata,HistBins)[0]
      bins=_np.histogram(xdata,HistBins)[1]
      bins=bins[0:-1]
      bins=bins+_np.mean(_np.diff(bins))
      
      #normalise pops
      pops=pops/float(_np.sum(pops))
      
      return bins,-_np.log(pops)
docs: Calculates the steady state potential. Used in 
      fit_radius_from_potentials.
  
      Parameters
      ----------
      xdata : ndarray
          Position data for a degree of freedom
      HistBins : int
          Number of bins to use for histogram
          of xdata. Number of position points
          at which the potential is calculated.
  
      Returns
      -------
      position : ndarray
          positions at which potential has been 
          calculated
      potential : ndarray
          value of potential at the positions above