WIP: OpenMP state #5, algebra #6 and resizer #7
State splits a given Range into an InnerState, one for each thread. The algebra's for_eachN calls for_eachN in parallel on each part, using the InnerState's algebra. There's an openmp_wrapper to parallelize the system function; this needs a way to pass on the offset.

The idea was that this design should allow using OpenMP on each MPI node with a single-threaded inner_state: mpi_state< openmp_state< inner_state > > with mpi_wrapper(openmp_wrapper(system_function))