Different results when A3D is run in Parallel

Imported from SVN. By Adrien Michel, Jun 5, 2018

Depending on the number of threads/core used, some differences appear in the results. This is not impacted by using OpenMP, MPI or both in hybrid mode, only the final number of threads used have an influence on the results.

Some plots are attached to show the difference between a procedural run (without parallelization), and runs with 4 threads (4 MPI, 4 OMP and an hybrid 2x2). This should be fixed to obtain the same output regardless the number of threads.