statically extracting nested structure to deduce a `BlockVector` type

This a support request.

In PDELab we have some magic to deduce the correct ISTL vector type, based on the blocking strategies listed in the Ordering of a GridFunctionSpace.

I would now like to support dune-functions bases in the PDELab backends (see pdelab/dune-pdelab!463). This means I have to reimplement that magic for a PreBasis. I'm currently unsure how to achieve this best. The PDELab implementation currently make heavy use of TypeTree::AccumulateType.Any suggestions are highly appreciated. In dune-functions @carsten.graeser implemented an ISTL backend, but here the vector type is user defined and the backend "only" implements the multi-index access.