[bugfix]Implement tuple protocol for custom type in resizetest.cc
This fixes some fallout of the removal of MultiTypeBlockMatrix::size():
By not providing a generic container interface you can no longer use generic
algorithms like indexed for-loops for matrices. One way to work around
this is to implement the tuple protocol to make Hybrid::size() work again.
For an alternative solution cf. core/dune-common!1209 (merged)