[blocks]

This pr adds support for spatially blocked discretizations (as used in the LRBMS) or systems of pdes. This is done by introducing `BlockVectorArray` and `BlockOperator` which are implementations of `VectorArrayInterface` and `OperatorInterface` that are composed of a collection of vector arrays or operators, respectively. In addition, this adds generalizations of some structures like extension algorithms or reductors. Everything is implemented in `playground/` and is supposed to merge cleanly to master.

Note that this pr is not yet intented to be merged, just to serve as an overview and to get automated builds.