Fixed serialization, added test for it.

* Also fixed to_string methods of operators, MpiSimulator and
  MpiSimulatorChunk.
Makefile builds mpi_sim_worker.
Minor clean up.

* Added worker executable to gitignore

* moved the practice scripts into a dir called scripts
SimulatorChunks on C++ side now work.

Chunks are now created on the C++ side, and correctly moved over to
the worker processes that are responsible for simulating them.

The whole process almost works, the data is getting in the right places.
Next step is to make the MPI simulation actually move forward.