Add to tutorials

[isaacsas]

• [x] conservation laws
• [ ] constant and BC species (explore network API interactions)
• [x] events
• [x] constraints
• [x] Showing how to use elimination of conservation laws to enable bifurcation detection.
• [x] How to unpack symbols and index solutions with them.
• [x] using begin and end in the DSL
• [x] Add Hodgkin-Huxley example from Juliacon.
• [ ] Dedicated units tutorial showing both DSL and symbolic interfaces.
• [ ] When appropriate cleanup and make visible MOL tutorial.
• [ ] Using array variables/parameters/species outside and in the DSL.

and from https://github.com/SciML/Catalyst.jl/issues/222

• [ ] Scaling to large reaction networks: handling parallelism and sparsity
• [ ] Choosing jump equation simulators
• [x] File I/O and reading in reaction networks
• [ ] Getting back Exprs from the systems, saving models?