Allow SBML import to a catalyst ReactionSystem

[sebapersson]

Currently models are imported as ModelingToolkit ODESystem - and we cover a relatively wide part of the semantic test-suite.

Naturally, it would be nice to have support for Catalyst ReacationSystem - at least for those cases where it is reasonable to have stochastic simulations in the model.

It should be relatively straightforward to achieve Catalyst support. Currently, the SBML model is parsed into a ModelSBML struct, which with other things stores the math for reactions, and their associated stoichiometry - which should be enough for translating efficiently into a Catalyst ReacationSystem.

As a starting example, it can be worthwhile to start with the first semantic test case, and see if the ModelSBML can be translated into a Catalyst ReacationSystem.