docs: def centre_mass_img(imdct):
      ''' Calculate the centre of mass of an image along each axes (x,y,z),
          separately.
          Arguments:
          imdct - the image dictionary with the image and header data.
          Output the list of the centre of mass for each axis.
      '''
      
      #> initialise centre of mass array
      com = np.zeros(3, dtype=np.float32)
      #> total image sum
      imsum = np.sum(imdct['im'])
  
      for ind_ax in [-1, -2, -3]:
          #> list of axes
          axs = range(imdct['im'].ndim)
          del axs[ind_ax]
          #> indexed centre of mass
          icom = np.sum( np.sum(imdct['im'], axis=tuple(axs)) \
                  * np.arange(imdct['im'].shape[ind_ax]))/imsum
          #> centre of mass in mm
          com[abs(ind_ax)-1] = icom * imdct['hdr']['pixdim'][abs(ind_ax)]
  
      return com