Feature: mix real-space density matrix for better convergence of DFT+U

[WHUweiqingzhou]

Fix #3535.

The list of changes

1. add a new input parameter mixing_dmr and corresponding UnitTests.
2. In charge_mixing.h, add a new member Base_Mixing::Mixing_Data dmr_mdata, whose memory can be allocated by Charge_Mixing::allocate_mixing_dmr(). In SCF, Charge_Mixing::allocate_mixing_dmr() is called in ESolver_KS_LCAO::eachiterinit() as

if (GlobalV::MIXING_DMR)
{
    const elecstate::DensityMatrix<TK, double>* dm
        = dynamic_cast<const elecstate::ElecStateLCAO<TK>*>(this->pelec)->get_DM();
    int nnr_tmp = dm->get_DMR_pointer(1)->get_nnr();
    this->p_chgmix->allocate_mixing_dmr(nnr_tmp);
}

3. In density_matrix.h, define a function DensityMatrix::save_DMR() store the current DMR before it is updated in ESolver_KS_LCAO::hamilt2density:

if (GlobalV::MIXING_RESTART > 0 && GlobalV::MIXING_DMR && iter >= GlobalV::MIXING_RESTART)
{
    elecstate::DensityMatrix<TK, double>* dm
        = dynamic_cast<elecstate::ElecStateLCAO<TK>*>(this->pelec)->get_DM();
    dm->save_DMR();
}

4. Similar to Charge_Mixing::mix_rho(chg), define a Charge_Mixing::mix_dmr(dm), and call it in ESolver_KS_LCAO<TK, TR>::eachiterfinish(int iter):

if (GlobalV::MIXING_RESTART > 0 && iter >= GlobalV::MIXING_RESTART && GlobalV::MIXING_DMR )
{
    elecstate::DensityMatrix<TK, double>* dm
                = dynamic_cast<elecstate::ElecStateLCAO<TK>*>(this->pelec)->get_DM();
    this->p_chgmix->mix_dmr(dm);
}

5. To avoid the failure of charge_mixing_test, I move many definitions of get-functions from .cpp into .h.

[dyzheng]

please add documentation for new parameters

[WHUweiqingzhou]

please add documentation for new parameters

done.