In tria.h, we use an object of type StraightBoundary<dim> to initialize a default...

In tria.h, we use an object of type StraightBoundary<dim> to initialize a default argument for a variable of type Boundary<dim>. Both of these types are only forward declared at this point. While most compilers seem to have no problem with this, ibm's xlC does as it tries to instantiate the default arg when seeing the explicit specialization declarations of member functions further down in the file.

While it may be that xlC is wrong, there is no need to be so contrived. We can just overload the set_boundary function and have one that has one and one with two arguments.

git-svn-id: https://svn.dealii.org/trunk@9537 0785d39b-7218-0410-832d-ea1e28bc413d