Same algorithm to be generalized

[Xiangyu-Hu]

The formulation on viscous force

SPHinXsys/src/shared/particle_dynamics/fluid_dynamics/viscous_dynamics.hpp

Lines 45 to 47 in a8a984c

	force += e_ij.dot((kernel_correction_(index_i) + kernel_correction_(index_j)) * e_ij) *
	mu_(index_i, index_j) * vel_derivative *
	inner_neighborhood.dW_ij_[n] * Vol_[index_j];

and that on diffusion

SPHinXsys/src/shared/particle_dynamics/diffusion_reaction_dynamics/diffusion_dynamics.hpp

Lines 100 to 103 in a8a984c

	const Vecd &grad_ijV_j = this->kernel_gradient_(index_i, index_j, dW_ijV_j, e_ij);
	Real surface_area_ij = 2.0 * grad_ijV_j.dot(e_ij) / r_ij_;
	Real phi_ij = gradient_species[index_i] - gradient_species[index_j];
	d_species += diff_coeff_ij * phi_ij * surface_area_ij;

are the same. Therefore, it would be better to generalize them into a single implementation