Fix copy-construction for LocalFunctions of DiscreteGlobalBasisFunctions

As mentioned in #20, the copy construction from a LocalFunction of a DiscreteGlobalBasisFunction is not usable as the LocalIndexSet is still bound to the LocalView of the original function. This fix lets the index set bind to the right LocalView. Since this binding would be undefined if the original function hasn't been bound to an element earlier, I had to introduce a flag bound_ that indicates if a given LocalFunction was bound already.

This MR does not touch the assignment operator yet, as it's broken on another level, see again #20.