Triangulation: implement global_*_cell_index_partitioner()

[peterrum]

... so that one does not have to cast the Triangulation object to parallel::TriangulationBase. CellAccessor has the necessary information independent of type of mesh.

[kronbichler]

I know we have this text also in the derived class, but I believe we typically use the style

Suggested change

	* Return partitioner for the global indices of the cells on the active
	* Return the partitioner for the global indices of the cells on the active

See also the documentation of the function set_mesh_smoothing below. (On the other hand, the one immediately preceding this does not use the article, so I am a bit split.)

[kronbichler]

I guess we never use this inside loops and rather only in basic circumstances? While the setup of an MPI_COMM_SELF partitioner should be cheap, it is not that cheap that we would like to do it millions of times.

[drwells]

I don't see how this can work - won't the object always get deleted immediately since weak_ptr won't keep it alive? For example, if I do

#include <memory>
#include <iostream>

std::weak_ptr<int>
get_42()
{
    return std::make_shared<int>(42);
}

int main()
{
    auto v = get_42();
    std::cout << "expired() = " << v.expired() << std::endl;
}

then the output is expired() = 1.

[drwells]

It looks like this also now works in 1D - nice!

[drwells]

/rebuild