move the slave dofs to the discrete function space (default)
This moved the slaveDofs to the discrete function space so that they can now be accessed using space.slaveDofs(). They are build automatically based on the sequence number of the space. In addition the old build() method returns a meaningful error message. dune/function/scalarproduct.hh still imports slavedofs.hh from the space folder to avoid problems with downward compatibility - could be removed after the release.
Corresponding branch in dune-fem-school-generator added to show required changes to dirichlet constraints class