[bugfix] Use correct integral type in the graph classes.

Parmetis has the option of using non-standard integral types
for the indices. In this case compilation failed. This is
fixed by using the type even for the graph representation
fed to metis/parmetis.

(cherry picked from commit 0ca78d5f)
Signed-off-by: Steffen Müthing <muething@dune-project.org>