Support additional backends

[david-zwicker]

It would be great if py-pde could also support additional backends for just-in-time compilation, e.g., jax or pytorch. Ideally, any of these packages (including numba) would be optionally, so we need to restructure our jit-implementation substantially. We could have general methods make_* that accept a backend argument, which then dispatches the call to a function in a special module (residing in a backends sub-package?). This would need to take place in multiple packages (pdes, grids, operators, boundaries, fields, ...), and we might need to consider inheritance and how we would like to deal with this in the implementations.

Taken together, this is a larger re-factoring, but it might be worthwhile to ensure py-pde stays relevant in the future.

[david-zwicker]

It might be best to define a lightweight custom plugin system, which defines hook functions (the make_* methods) that can be accelerated. We can then provide additional modules that provide suitable hook implementations. These modules would only be loaded if the respective packages (numba, jax, ...) are actually available, which can presumably be handled with the config object.