Add support for alternative automatic differentiation backends

[matt-graham]

Adds support for JAX and SymNum as alternative automatic differentiation backends to the current Autograd fallback.

All systems now expose a backend argument in their initializer that can be used to specify name of backend to use, defaulting to None, corresponding to not using an automatic differentiation backend.

Also adds a vjp_and_value differential operator to Autograd wrapper which was previously missing.

Todo

• Update README with details of autodiff backends
• Rename autodiff optional dependencies group to autograd
• Add new optional dependencies groups for jax and symnum
• Add tests for autodiff backends