RunPlanVector file IO

[Robadob]

Currently just an (untested) minimal viable product

Still required for merge

• [ ] Throw exception if steps=0 is loaded, and model doesn't have an exit condition.
• [ ] XML version
• [ ] Integration with CUDAEnsemble config/arg parsing
• [ ] Export import the RunPlanVector 's internal random state?
• [ ] Testing
• [ ] Python interface testing
• [ ] Documentation

Relates: issue #440, discussion #1229