encode full file path in the single-inclusion macro
Move the MultiYGrid class right in front of the YaspGrid class

To prepare for merging
Test operator<< for YaspGrid
[cleanup] remove out-commented code
[cleanup] Implement operator<< for YaspGrid directly

Previously it was implemented for the deprecated base class
MultiYGrid.  We also remove one level of redirection.
[cleanup] Start merging the MultiYGrid constructors into YaspGrid
[cleanup] Move MultiYGrid constructors (almost entirely) into YaspGrid
[cleanup] Merge method MultiYGrid::coarsen into YaspGrid
[cleanup] Make MultiYGrid inherit from GridDefaultImplementation

Previously it was YaspGrid which inherited from both MultiYGrid
and GridDefaultImplementation.  This patch avoids the multiple
inheritance.
[cleanup] Rename template parameters of MultiYGrid to match those of YaspGrid

To ease later merging.
[cleanup] Merge method MultiYGrid::refine into YaspGrid
[cleanup] Really do merge the MultiYGrid class into YaspGrid

MultiYGrid existed only for historical reasons.  Today it just serves
to confuse maintainers.  Hence away with it!
[cleanup] Remove obsolete typedefs IS and YGLI
[bugfix] spelling
[cleanup] Remove two minor pointer indirections
implement boundaryId only if DUNE_GRID_EXPERIMENTAL_GRID_EXTENSIONS is set
[cleanup] remove a few this->
[cleanup] remove unused methods rbegin/rend
[cleanup] Use ReservedVector to store the level grids

Instead of a C array and a separate counter.
[cleanup] Remove the MAXL parameter

It is not used consistently anyway