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?

[ranocha]

We could generalize compose to colored trees and their associated series to deal with this. Right now, we only have substitute for series based on colored trees. To implement compose, we need to implement splittings of colored trees in RootedTrees.jl, in particular the SplittingIterator.