This is a major rewrite of Dune::YaspGrid. It now
- has full entities of all codimensions
- communicates on all these codimensions
- works for arbitrary dimension (again)

The performance implications in comparison to the old
code have been measured. While iterating over the grid
slowed down 50%, iterating over the intersections of the
grid sped up by around 50%. The slowdown in iteration is
not surprising, as an additional wrapping class (the YGrid)
is introduced in order to implement the new features.
Iterating over the grid is still very fast, taking about
15 CPU cycles per cell in a test run.

Here are some implications for the implementation
(not of interest for users):
- THe former YGrid is now called YGridComponent
- A YGrid is a collection of YGridComponent. A YGrid
  describes the parts of a grid given by partition type
  and a codimension. It offers iterators to iterate over
  the multiple components. YGridList does the same thing
  for send and receive lists.
- Yasp handles codimensions generically now. The former
  data structures {cell,vertex}_{...} are replaced by
  arrays of size dim + 1 of {...}.
- The entity implementation provides a generic implementation
  of the index methods according to the new generic reference
  elements.
- A namespace Yasp is introduced and used to hide away some
  free functions from the user.
- YaspGeometry now handles the bitsets required by
  AxisAlignedCubeGeometry directly.

The incode and doxygen documentation of the rewritten code
was improved whenever it seemed necessary.