Instead of implicitly assuming that all data has been recieved after
the child process terminates, verify that it matches the expected
length. This changes the signals between processes to 1) end_of_sim
and 2) end_of_data:[#bytes]. Upon completion verify that the length
of the base64 byte string matches this number.

Turns out that padding is necessary. Added back code to only add the
minimal amount of padding (e.g. '=').

Refactor mpi_child.py into a proper Python class.

Also reuse _clone_and_simulate() between backends.

- add skip_MPI_import to MPISimulation
- refactor MPIBackend to separate _process_child_data for testing

Still raise a custom exception for troubleshooting if it does arise

Use the parameter prng_seedcore_initial