Unit validation and tests

[isaacsas]

I also added a show definition for Reactions that prints out stuff like

Reaction{Any, Int64}: γ, A + B --> 2B

so a user could easily round-trip to/from the DSL. (I kept the summary with type info, but could drop that from show if anyone thinks that makes sense.)

[isaacsas]

@lamorton could you check that I haven't messed anything up w/regards to the MT plans for validation? I changed how it works a bit at the level of a Reaction, and then added a version that works on the whole ReactionSystem (which is what is now checked in the ReactionSystem constructor).

[lamorton]

@lamorton could you check that I haven't messed anything up w/regards to the MT plans for validation?

It looks OK to me. I'm unsure exactly how things will evolve on the MT side, but it should be fine if you have specialized the validation to suite your needs here.