Properly handle RandomStates #99
Description
RandomState should be managed so that:
- separate instances are used by different entities (submodels, species population) so changes in the behavior of one doesn't influence the behavior of another
- simulations can be restarted from checkpoints containing the random state
When this has been done, TestDynamicModel.do_test_expression_dependency_dynamics() no longer needs to temporarily suspend rounding by DynamicSpeciesState.get_population() so that it doesn't use the RandomState it shares with submodels