Feature/fix parmetisgridpartitioner compilation

While running the dune-grid tests for the 2.4.1 release, I stumbled over
`test-loadbalancing`, which stubbornly refused to work on Ubuntu 14.04.
This merge request fixes two things:

- It makes the partitioner work with ParMETIS 3.x by adding the
  necessary typedefs for `idx_t` and `real_t`.
- It completely disables the partitioner when running with scotch,
  because scotch doesn't emulate all of the functions required by it.

See merge request !34
(cherry picked from commit 616551eb

)

Signed-off-by: Steffen Müthing <muething@dune-project.org>