Jax and Numpy call priority

Hi,

I was wondering whether it was possible to force the use of numpy even though jax was installed. Namely, I would like to run some parts of my code with numpy, while other parts should still be run with the jax backend.

[This line](https://github.com/Farama-Foundation/Jumpy/blob/main/jumpy.py#L24) seems to suggest that it globally determines what backend is used. I could change the `_has_jax` variable before and after, but it seems a hacky.

If there is a better way, let me know!

A suggestion could be to use a context manager for this, similar to how `chex.fake_jit` works:

```
import jumpy as jnp

with jnp.use_jax():
....
```