In particular this means having to carry mappers for the local indices around.
This makes the code more verbose, but it also makes it easier to see what is
happening.  While coding this stuff I keep having doubts about whether all
the renumbering works correctly...

[[Imported from SVN: r9925]]

[[Imported from SVN: r9923]]

The local copy was to get the thing developed and working.  Now that it is
working we upstream it into dune-grid.

[[Imported from SVN: r9922]]

The IndexSet interface class from dune-grid doesn't have it, and we want
to be like the interface method as much as possible.

[[Imported from SVN: r9921]]

That's what this method is called in the dune-grid IndexSet
interface.

[[Imported from SVN: r9919]]

[[Imported from SVN: r9918]]

[[Imported from SVN: r9901]]