docs: def full_load_hours(network, boundaries=[], filename=None, two_cb=False):
      """Plot loading of lines in equivalten full load hours.
      
      Parameters
      ----------
      network: PyPSA network container
          Holds topology of grid including results from powerflow analysis
      filename: str or None
          Save figure in this direction
      boundaries: array
         Set boundaries of heatmap axis
      two_cb: bool
          Choose if an extra colorbar for DC-lines is plotted
      
      """
      cmap = plt.cm.jet
  
      array_line = [['Line'] * len(network.lines), network.lines.index]
  
      load_lines = pd.Series(abs((network.lines_t.p0.mul(
                                  network.snapshot_weightings, axis=0).sum() /
                                  (network.lines.s_nom))).data, index=array_line)
  
      array_link = [['Link'] * len(network.links), network.links.index]
  
      load_links = pd.Series(abs((network.links_t.p0.mul(
                                  network.snapshot_weightings, axis=0).sum() /
                                  (network.links.p_nom))).data, index=array_link)
  
      load_hours = load_lines.append(load_links)
  
      ll = network.plot(line_colors=load_hours, line_cmap=cmap, bus_sizes=0,
                        title="Full load-hours of lines", line_widths=2)
  
      if not boundaries:
          cb = plt.colorbar(ll[1])
          cb_Link = plt.colorbar(ll[2])
      elif boundaries:
          v = np.linspace(boundaries[0], boundaries[1], 101)
  
          cb_Link = plt.colorbar(ll[2], boundaries=v,
                                 ticks=v[0:101:10])
          cb_Link.set_clim(vmin=boundaries[0], vmax=boundaries[1])
  
          cb = plt.colorbar(ll[1], boundaries=v,
                            ticks=v[0:101:10])
          cb.set_clim(vmin=boundaries[0], vmax=boundaries[1])
  
      if two_cb:
          cb_Link.set_label('Number of full-load hours of DC-lines')
          cb.set_label('Number of full-load hours of AC-lines')
  
      else:
          cb.set_label('Number of full-load hours')
  
      if filename is None:
          plt.show()
      else:
          plt.savefig(filename)
          plt.close()