Proper IR

[harpocrates]

This is a (mostly internal) rewrite for making the implementation match closely the specification https://galoisinc.github.io/AMIDOL/documentation/ir.

Highlights

• the IR supports input/output predicates
• palette elements (nouns & verbs) can involve multiple states & events (for instance: we can have a population noun which comes with a birth and death process)
• the VDSOL is turned into IR by mapping over nouns/verbs to their respective underlying IR model and composing all of these models together

Additionally added support for:

• extract an IR model from a Julia AST
• a Julia Gillespie backend

Broken

Some things that broke or are loose ends to fix (after this PR is merged):

1. backends don't do anything with input predicates, but they should (this should be easy to fix)
2. the PyCeS backend was broken and commented out
3. the SciPy linear steady state backend is broken (although this one should probably just be junked)
4. the Julia Gillespie backend takes a while to run - the browser even re-POSTs the request because it takes so long!

[harpocrates]

/cc @ewdavis This should be roughly what I demo-ed earlier today, but with the examples directory unbroken. Note the final caveat in my summary: the Julia backend takes too long to run and the frontend re-posts the same query (the end result is that no plots ever make it out). I'll fix that first thing next week.

There's probably some other small things I broke, but I'll fix those as they appear.