implement a generic IntersectionMapper class

this class takes an arbitrary dune grid view and makes it possible to
map intersections to a unique, grid-view-global index, where it does
not matter which element is to the interior and which is to the
exterior of the intersection.

For conforming grids, this class provides the same functionality as a
mapper for faces (albeit using a different ordering), but
IntersectionMapper also works for non-conforming grids which only
provide intersections...