Zheng Invariants full implementation

This branch currently differs from master only in the specification and implementation of invariants.py; my implementation handles both the scalar and form invariants correctly, and currently works for all continuous 3d symmetries (e.g. isotropy, hemitropy, all types of transverse isotropy, etc), some discrete 3d symmetries ( crystal groups), and some 2-d symmetries. I'm currently working on adding more symmetries to complete the tables. Also, I use JAX instead of autograd (so we can JIT compile all the invariants and still differentiate them, as well as run on GPU/TPU will no code modifications), so this branch will also be for moving the current autograd code into JAX. (@jedbrown the code that's in the private git repo I shared with you is now in crikit/invariants.py --- while I'm working on adding more symmetries, I'll also turn that file into a more readable module of its own, and put the tests in the test directory, but until invariants.py becomes a directory (invariants), the README in that repo is still accurate w.r.t. line numbers)