add tests + exception for MPI runs

[ltalirz]

Will merge #47 since it is needed by aiida-lsmo, but one should add

• a test of a run withmpi=True
• if possible, an exception/warning, if a number of processes >1 is detected (or implement a solution for it)

[greschd]

or implement a solution for it

Maybe this can be done with mpi4py? Something like:

• process 1 checks for a cache hit, all other processes wait
• if cache hit, use cache and terminate
• if no cache hit, each process launches its "real executable"?

Not sure if this would work, but might be worth doing a PoC for.

[ltalirz]

Thanks for the suggestion @greschd, yes I guess that could work!

In my view, it does look a bit convoluted though, and perhaps it is better to go the monkeypatching route #49.
Besides making the handling of MPI codes straightforward, it also has the key benefit of improving debugging info when the call to the mock executable does not go as expected (which, in my experience, is the main pain point in using mock_code at the moment, and hinders its wider adoption).