Move create_union_triangulation() and extract_boundary_mesh().

These functions were previously in GridTools, but conceptually they create
meshes and so should be in GridGenerator. Move them there but leave the
old functions in GridTools with a note that they are now deprecated.

Also structure the documentation of the functions in GridGenerator better.
Merge pull request #103 from bangerth/move-functions

These functions were previously in GridTools, but conceptually they create
meshes and so should be in GridGenerator. Move them there but leave the
old functions in GridTools with a note that they are now deprecated.

Also structure the documentation of the functions in GridGenerator better.