Completely reworked the pull request.

Completely reworked the pull request.
The particle handler now has a GridTools::Cache that is used to obtain the vertex_to_cell_centers and vertex_to_cells
This enables the same feature as before but is a lot cleaner and flexible.
However, the cache has to be stored in a unique pointer because the particlehandler has a constructor that enables it to be constructed without a triangulation.
This is not the case for the GridTools::Cache. Consequently, I thought that unique an std::unique was a good compromise here.

As always, looking forward to your comments :)!