[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().
Copy global_cell instead of casting.

By doing this there will be no lifetime interference between the
adapted CpGrid and the stored UnstructuredGrid.
Detect if cells are all active and set up UnstructuredGrid to reflect this.
Copy cartesian dimensions again.

This was accidentally removed in one of the last patches.
Cater case where grid is not a CpGrid.
Merge pull request #70 from blattms/fixes_global_cell_of_GridAdapter_c_grid

[bugfix] Initializes global_grid pointer of UnstructuredGrid