Adds missing includes for SGrid and std::accumlate.

I really wonder how this could ever have compiled for others.
[cmake] Export DUNE_GRID_EXAMPLE_GRIDS_PATH for use in downstream modules

This variable was previously not available to downstream modules. This prevented them
from using the grid examples within dune-grid for testing. Now we set this variable
in the package configuration file and it can be addressed in the CMake build system.