[bugfix] fix typo in namespace
The namespace Functions in optional.hh has been erroneously renamed to Function in !53 (merged).
I found this by chance as it clashes with the definition of the class Function from dune-common.
The namespace Functions in optional.hh has been erroneously renamed to Function in !53 (merged).
I found this by chance as it clashes with the definition of the class Function from dune-common.