symmetrize_rho slow on large systems for OFDFT

[Cstandardlib]

Details

When calculating with esolver ofdft, a large part of total time is consumed by Symmetry::rhog_symmetry (up to 90%+ for large systems).
Symmetry::rhog_symmetry is called exactly once in ESolver_OF::before_opt:

    Symmetry_rho::symmetrize_rho(PARAM.inp.nspin, this->chr, this->pw_rho, ucell.symm);

    for (int is = 0; is < PARAM.inp.nspin; ++is)
    {
        if (PARAM.inp.init_chg != "file")
        {
            for (int ibs = 0; ibs < this->pw_rho->nrxx; ++ibs)
            {
                // Here we initialize rho to be uniform,
                // because the rho got by pot.init_pot -> Charge::atomic_rho may contain minus elements.
                this->chr.rho[is][ibs] = this->nelec_[is] / ucell.omega;
                this->pphi_[is][ibs] = sqrt(this->chr.rho[is][ibs]);
            }
        }
        else
        {
            for (int ibs = 0; ibs < this->pw_rho->nrxx; ++ibs)
            {
                this->pphi_[is][ibs] = sqrt(this->chr.rho[is][ibs]);
            }
        }
    }

When init_chg is set by default, i.e. the first if-branch, symmetrize_rho will be called anyway and the chr.rho is overwritten immediately.

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