Returns the calculated register of the coiled coil and the fit quality.

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:]