Draft: Power composite with gridview

As discussed there are setups, where the children of composite(...) or power(...) do not have a unique GridView, e.g. when doing cut-cell simulations, where the different space correspond to spaces on different subdomains. The CompositePreBasis and PowerPreBasis currently return the GridView of the first child, which can't be the correct answer. Basically the user has to specify what the correct GridView is in this case and thus it should be handled directly by composite(...) or power(...).