Add feature to figure out independent tracers and use it

[briochemc]

Assuming one wants to simulate two independent tracers at the same time, there is no need to construct the full matrix. A better approach is to split the model into separate sub-models. Maybe a good solution is to raise a warning if some tracers are independent. The user can then think of it as separate models with separate inputs and outputs, that can be combined after the fact if needed.