dev: Type promotion.

[daniel-dodd]

The types returned are inconsistent over operations.

Currently adding LinearOperators to each other return and linear operator. Also adding a jax.numpy.ndarray to a linear operator returns a LinearOperator.

But matrix multiplication does not return a LinearOperator. It returns an jax.numpy.ndarray.

The question is: How should types be coerced?

Should we always return a linear operator? Should operations involving a jax.numpy.ndarray, instead, always return a jax.numpy.ndarray - since these are the least structure preserving.