[common.memory] allow move of ConstStorageProvider

I see no harm in it atm.
[functions.constant] added unit_matrix()

This can be used to create the appropriate unit element wrt multiplication.
[functions.interfaces] added default implementation for deprecated copy()

Thus new functions do not have to implement this.
[grid.walker] added static check

This checks, if a given grid view is really a Dune::GridView (or a
Dune::Fem::GridPartInterface, which is also fine).