added size methods and index methods.

After talking with Rbert I moved the access to the indices into a seperate
class (here YaspIndex). Thus we can change and add indices more easily
in the future (although I would not overuse this fact). Moreover, the
grid can build up the indices "on demand" only if they are requested.

If you want to see a mapper then look into dune-dd/mapper.hh

[[Imported from SVN: r1960]]