Virtual files are dumped to disk at each iteration in MPI processes, drastically reducing memory usage

- For safety, since some loops are murky, overallocate some arrays of size nspheres instead of configurations
- correct a race in the definition of myompthread
- add some omp barriers to force synchronisations

- turn C4 from a structure to a class (to do the above), move its initialisation to its constructor
- eliminate "allocate_vectors()", redundant and including a (wrong) double allocation