docs: def ppm_idx(f_ppm, lb, ub):
      """
      Create a slice object according to the ppm scale
  
      Parameters
      ----------
      f_ppm : float array
          The frequency bins (in the ppm scale). Assumed to be descending
  
      lb,ub : float
          The lower/upper bounds for indexing
          
      Returns
      -------
      idx : slice object which can be used to index 
      """
      idx0 = np.argmin(np.abs(f_ppm - lb))
      idx1 = np.argmin(np.abs(f_ppm - ub))
      return slice(idx1, idx0)