minor cleanup: use Dune::array<int> instead of FieldVector<int>
Remove unused method

Thanks to Dominic Kempf for pointing this out
remove out-commented code
remove more out-commented code
minor coding style fixes
Implement a dedicated LeafIndexSet class, instead of using the max-LevelIndexSet

Previously, in a rage of cleanup, I had removed the class that implements
the LeafIndexSet for YaspGrid.  Instead, the LevelIndexSet for the maxLevel
was used.  However, that creates a problem when a LeafIndexSet is copied,
and then the max level removed from the grid.  In that case, the copied
LeafIndexSet points to a level that is no longer existent.  This lead to
crashes in adaptivefinitevolume (in dune-grid-howto), see FlySpray #1276.

This patch puts the separate LeafIndexSet class back in.
Coding style: 'if'-clause and conditional statement not on a single line
remove out-commented debugging output
Bugfix: do not renumber vertices when load balancing with only one process

When there is only one process, then calling loadBalance will not modify
the grid (as expected).  However, in some cases the vertex and element
indices are changed.  This patched prevents that.

This fixes FS 1353.
Merge branch 'master' of http://git.dune-project.org/repositories/dune-grid