docs: def heptad_register(self):
          """Returns the calculated register of the coiled coil and the fit quality."""
          base_reg = 'abcdefg'
          exp_base = base_reg * (self.cc_len//7+2)
          ave_ca_layers = self.calc_average_parameters(self.ca_layers)[0][:-1]
          reg_fit = fit_heptad_register(ave_ca_layers)
          hep_pos = reg_fit[0][0]
          return exp_base[hep_pos:hep_pos+self.cc_len], reg_fit[0][1:]