Fix serialization of dof handlers

 Fix an issue reported by David Wells.

The problem here is that class DoFHandler only uses pointers to the
DoF faces and levels objects in the .h file, so we thought that we
don't have to #include the respective header files where these classes
are declared. But we also have a serialization function that uses
them and if you call it, the compiler will complain about undeclared
classes being used.