Return SRTs from SpinReveralTransformComposite

It would be quite straightforward to add a `return_transforms=False` kwarg. The only difficult part is deciding what to return - we could obviously return [the SRT matrix](https://github.com/dwavesystems/dwave-preprocessing/blob/b3c4a9240d93d7237595a294dbb82c7e6c5fe2c8/dwave/preprocessing/composites/spin_reversal_transform.py#L155), but we would probably want to indicate which variables are matched to which columns somehow.