Add custom concatenate/stack functions

[calbach]

This is to support stacking / concatenating arrays within a sympy expression.

For discussion:

• Does this belong in sympy2jax, or would it be better if we specified this as a custom_func (as a client)?

[patrick-kidger]

Looks like the pre-commit checks failed.

FYI you can arrange to have these run automatically whenever you commit. They will either pass and commit, or fail and autoformat.

If the latter, you can then check you're happy with the autoformat, followed by git add -u; git commit to add the changes and commit for real.

(See CONTRIBUTING.MD.)

[calbach]

Fixed, I missed this repo was using isort. I'll try out the precommit hook next time.

[patrick-kidger]

Merged! Ty.