Add constrained entries to partitioner_coarse in NonNestedMG

[fdrmrc]

The following error

--------------------------------------------------------
An error occurred in line <861> of file </workspace/dealii/include/deal.II/base/partitioner.h> in function
    unsigned int dealii::Utilities::MPI::Partitioner::global_to_local(dealii::types::global_dof_index) const
The violated condition was: 
    in_local_range(global_index) || is_ghost_entry(global_index)
Additional information: 
    Global index 302 neither owned nor ghost on proc 5.

is triggered by:

dealii/include/deal.II/multigrid/mg_transfer_global_coarsening.templates.h

Lines 3907 to 3911 in f6b833b

	constraint_info.read_dof_indices(i,
	numbers::invalid_unsigned_int,
	cell,
	constraint_coarse,
	this->partitioner_coarse);

when running with locally refined and a p::d::T triangulations. The present PR adds the missing constrained entries to the coarse partitioner (and a test with a locally-refined meshes)

[peterrum]

? I guess this should be in the .h file?

[fdrmrc]

The issue is that MGTwoLevelTransferImplementation::create_coarse_partitioner() is defined in .templates.h and is private. Some compilers are also not happy with the present PR, so I moved it to public members and removed the friend declaration. Do you agree with that? I don't know what would be the best option

[peterrum]

/rebuild