Add stubs for jax.ArrayN.{min,max,std,mean}

[mrahtz]

In JAX, array classes have methods .min(), .max(), .std() and .mean(). We should provide stubs for these.

[hylkedonker]

Would it also be possible to add .sum() to the list? I currently use jnp.sum(X) instead of X.sum() to infer the right index semantics. Ideally, I prefer to use X.sum() instead of jnp.sum(X), but this raises mypy errors such as:

error: "ArrayN[DType, ...]" has no attribute "sum"  [attr-defined]