Add pre-commit hooks for unused imports

Fix sympy2jax for rational numbers

Add mappings from sympy exact One/Half

Add export key error telling user to set function mappings

Propagate and check torch/jax mappings

Refactor unnecessary `else` / `elif` when `if` block has a `return` statement

Format code with black

Fix sympy2jax for selection of indices

Allow custom selection of X matrix in torch/jax modules

Refactor lazy loading of torch and jax

Add torch format output; dont import jax/torch by default

Move JAX export to separate file