How to handle re-normalization of B-series

[ranocha]

Internally, B-series store the coefficients without division by symmetry(t). Sometimes, it can be useful to perform this division explicitly and just use the re-normalized series. However, there are some issues with this since all interfaces assume the default normalization. Some options could be

1. Just renomalize! and tell everyone that they need to be careful.
2. Return an array of normalized coefficients - but we lose the direct association with trees in this case.
3. Add another field to the existing implementation(s) of B-series like normalized::Bool defaulting to true. Check this field whenever we perform operations on the B-series and throw an error if required. Change this field to false when renormalize! is called. The function renormalize! could switch between both modes.
4. Tell people that they need to renormalize the coefficients themselves. That's essentially what we are doing right now, e.g., in

BSeries.jl/src/BSeries.jl

Lines 361 to 366 in 04b2f99

	!!! note "Normalization by elementary differentials"
	The coefficients of the B-series returned by this method need to be
	multiplied by a power of the time step divided by the `symmetry` of the
	rooted tree and multiplied by the corresponding elementary differential
	of the input vector field ``f``.
	See also [`evaluate`](@ref).

See #117 (comment)