Anisotropic diffusion

[Xiaojingtang1234]

This is for the anisotropic diffusion coupling with anisotropic resolution.

• variables vec6d, mat6d are implemented in base_data_type.h
• anisotropic_diffusion_relaxation.hpp is created for template classes.
• AnisotropicKernelCorrectionMatrix is for the further kernel correction.
• AnisotropicDiffusionRelaxation is for the anisotropic diffusion relaxation.*Laplacian_x, *Laplacian_y, are recorded for checking the diffusion rate in different directions.
• The results are validated with functions $$\Phi = x +y $$ and $$\Phi = x^2 +y^2 $$.

Note that two issues related are created.
1, The displacement issue should be implemented another real rij_vector. . Related to #742
2, The current 2d case is depend on anisotropic_diffusion_relaxation.hpp . Related to #743

Additionaly, the neighborhood statement should be modified to if (kernel_->checkIfWithinCutOffRadius(displacement) ), matching the one in NeighborBuilderInner, note that index_i != index_j should be deleted, considering index maybe the same in different bodies.

void NeighborBuilderContact::operator()(Neighborhood &neighborhood,
                                        const Vecd &pos_i, size_t index_i, const ListData &list_data_j)
{
    size_t index_j = list_data_j.first;
    Vecd displacement = pos_i - list_data_j.second;
    Real distance = displacement.norm();
   if (kernel_->checkIfWithinCutOffRadius(displacement) )
    {
        neighborhood.current_size_ >= neighborhood.allocated_size_
            ? createNeighbor(neighborhood, distance, displacement, index_j)
            : initializeNeighbor(neighborhood, distance, displacement, index_j);
        neighborhood.current_size_++;
    }
};

[Xiangyu-Hu]

@Xiaojingtang1234 we can have a discussion on the implementation this week.