feat: Kronecker linear-operator.

[daniel-dodd]

Would be nice to support a Kronecker linear operator, that takes a list of linear operators and constructs a Kronecker matrix. This would be highly useful for spatial statistics.

import jax.numpy as jnp

mat = jnp.array([[1., 2.], [3., 4.]])
diag = jnp.array([5., 6.])


A = DenseLinearOperator(mat)
B = DiagonalLinearOperator(diag)

KroneckerLinearOperator([A, B]) # A ⊗ B