[function.interfaces] mark copy() as deprecated

Did not make much sense in the end, Rene was right after all^^
[common.memory] added ConstStorageProvider
[functions.interfaces] reorderen includes
[functions.interfaces] do not guard dune-geometry usage
[functions.combined] complete rewrite

There is now a generic combined function and the combination action can be
defined by specializing a small internal class. The old Difference is now
implemented as a derived class.
[functions.combined] added Sum
[functions.combined] added dimensions for sum
[functions.interfaces] added operator+