[bugfix] Initializes global_grid pointer of UnstructuredGrid

Previously access global_grid of the UnstructuredGrid returned by
c_grid() was erroneous. This patch fixes this by simply reseting the
pointer of UnstructuredGrid::global_cell to CpGrid::global_cell.
Note that we silently assume that the template parameter Grid is a
CpGrid!

Fixes issue #69
[bugfix] Fixes missing initialization of UnstructuredGrid::cartdims in GridAdapter.

UnstructuredGrid::cartdims was never initialized in GridAdapter. This
patch fixes this by copying the values from CpGrid::logicalCartesianSize().