Feature/convergence test

Add example Python script that runs convergence tests.

Currently a convergence test for fields is present.

TODO: add particle convergence test (draft status can be removed afterwards).

cpp/full_code/NSVEparticles.cpp

@@ -98,6 +98,7 @@ int NSVEparticles<rnumber>::initialize(void)
         this->particles_output_writer_mpi->close_file();
     }
 
+    DEBUG_MSG("fs->iteration = %d\n", this->fs->iteration);
     DEBUG_MSG_WAIT(MPI_COMM_WORLD, "about to call particles_system_builder\n");
     this->ps = particles_system_builder(
                 this->fs->cvelocity,              // (field object)