Making cache-friendly loops

[FabienPean-Virtonomy]

https://github.com/Xiangyu-Hu/SPHinXsys/blob/1e0042b5040c6f59b9fa98e7634c33f63090fd1a/SPHINXsys/src/shared/particle_dynamics/solid_dynamics/solid_dynamics.cpp#L770-L778

While considering vectorization and see how it could fit, I ended up thinking specifically about this part of the code above. This kind of loop is suboptimal as it is doing one action on several different arrays. A data-oriented and cache-friendly approach would be applying the operation for each array one after the other:

for(auto index_i = 0; index_i < total_real_particles; ++index_i)
   	pos_n_[index_i] += vel_n_[index_i] * dt * 0.5;
for(auto i = 0; i < total_real_particles; ++i)
	F_[index_i] += dF_dt_[index_i] * dt * 0.5;
for(auto i = 0; i < total_real_particles; ++i)
        rho_n_[index_i] = rho0_ / det(F_[index_i]);
for(auto i = 0; i < total_real_particles; ++i)
	stress_PK1_[index_i] = F_[index_i] * material_->ConstitutiveRelation(F_[index_i], index_i);

It does not seem so widespread overall in the codebase (as only one field is getting touched) but the architecture around ParticleDynamics1Level in the library favors this kind of pattern.

[Xiangyu-Hu]

It is good starting point. Need some test to see whether we get benefits. And how to combine with parallelization.