docs: def get_grid(key):
      """Return a :class:`crtomo.grid.crt_grid` instance, with a debug grid that
      is distributed with the crtomo package. Multiple grids are available:
  
          * key=20 - a 20 electrode grid with 1m spacing, 4 elements between
            electrodes, rectangular elements.
          * key=40 - a 40 electrode grid with 1m spacing, 4 elements between
            electrodes, rectangular elements.
  
      Parameters
      ----------
      key: string
          key that identifies the grid
  
  
      Returns
      -------
      grid: :class:`crtomo.grid.crt_grid` instance
          loaded grid object
  
      """
      rbase = grid_files[key]
      elem_file = pk.resource_filename('crtomo', rbase['elem'])
      elec_file = pk.resource_filename('crtomo', rbase['elec'])
  
      grid = CRGrid.crt_grid(elem_file=elem_file, elec_file=elec_file)
      return grid