Speed-up MCMGMapper using arrays, cf. FS#854.

Patch by Martin, I fixed the failing patch hooks and some white space
changes.
[docu] Fix typo and small documentation issue.
Clean up mcmgmappertest.cc.

Remove dead code.
Whitespace changes.
Reformat comments.
Improve code coverage in mcmgmappertest.

Add test of vertice mapping, map(e, i, codim) is now tested, too.
Add test of multiple codimensions: mapper for elements and edges.
Change MCMGMapper internals to use GlobalGeometryTypeIndex instead of magic.

This follows the suggestions from Carsten and Steffen.
See FS#854 for more details.
Fix last-minute changed variable name issue.
Merge remote-tracking branch 'origin/master' into p/gruenich/fs854

Assert we calculate the index only for subentities that are supported by the
MCMGMapper instance (similar to commit ...5311378818).

Conflicts:
	dune/grid/common/mcmgmapper.hh
Merge remote-tracking branch 'origin/master' into p/gruenich/fs854