Figure out how to call an ASCOT5 MPI run from within a single process of the MOOSE app.

[bielsnohr]

Some more detailed thoughts are provided here: #3 (comment)

It was agreed that option 1 seems to be the best course forward. However, there is one alternative not mentioned there: manually split the input to ASCOT5 and then run one of the partitioned groups of ions on each MPI process. This is similar to what ASCOT5 is actually doing itself when it runs in parallel! This is enabled by the fact that simulated ions are assumed to be independent of each other (i.e. no collisions or interactions between them).