Composition with different step sizes and different right-hand-sides

[ketch]

A potential use case involves finding B-series for operator splitting methods. To do this, we would need composition to work with different step sizes (this should be easy) and different generic right-hand sides; e.g. abstract symbolic functions f and g.

The substeps of the splitting method might be solved exactly or with a RK method.

@ranocha do you have any suggestions about how to implement this?