Cleanup the Vector::thread_loop_partitioner logic.

[drwells]

It was previously possible to, in one function call, either set up the partitioner multiple times or for inappropriately small vectors. This commit cleans up the way we handle the partitioner in multiple places so that, when possible, vectors share partitioners and do not set up partitioners if they are too small.

Fixes #7646.

[drwells]

/rebuild

[kronbichler]

This looks good to me. Is there anything in la_parallel_vector.h that can/should be treated similarly?

[masterleinad]

We have a number of tests failing

4207 - multigrid/transfer_block_select.debug (Failed)
4220 - multigrid/transfer_prebuilt_02.debug (Failed)
4221 - multigrid/transfer_system_05.debug (Failed)
4223 - multigrid/transfer_01.debug (Failed)
4231 - multigrid/transfer_system_adaptive_07.debug (Failed)
4235 - multigrid/transfer_compare_01.debug (Failed)
4245 - multigrid/transfer_03.debug (Failed)
4256 - multigrid/transfer_block.debug (Failed)
4259 - multigrid/transfer_system_adaptive_08.debug (Failed)
4261 - multigrid/transfer_system_04.debug (Failed)
4267 - multigrid/transfer_system_adaptive_01.debug (Failed)
4272 - multigrid/transfer_system_adaptive_03.debug (Failed)
4274 - multigrid/transfer_system_02.debug (Failed)
4277 - multigrid/transfer_system_adaptive_09.debug (Failed)
4280 - multigrid/transfer_prebuilt_01.debug (Failed)
4283 - multigrid/transfer_system_adaptive_04.debug (Failed)
4288 - multigrid/transfer_system_03.debug (Failed)
4289 - multigrid/transfer_system_adaptive_02.debug (Failed)
4294 - multigrid/transfer_prebuilt_03.debug (Failed)
4299 - multigrid/transfer_02.debug (Failed)
4305 - multigrid/transfer_select.debug (Failed)
4306 - multigrid/transfer_system_01.debug (Failed)

with the error

multigrid/transfer_system_01.debug: RUN failed. ------ Additional output on stdout/stderr:


--------------------------------------------------------
An error occurred in line <186> of file </jenkins/workspace/dealii_PR-7683/include/deal.II/lac/vector_operations_internal.h> in function
    void dealii::internal::VectorOperations::parallel_for(Functor&, dealii::internal::VectorOperations::size_type, dealii::internal::VectorOperations::size_type, const std::shared_ptr<dealii::parallel::internal::TBBPartitioner>&) [with Functor = dealii::internal::VectorOperations::Vector_set<double>; dealii::internal::VectorOperations::size_type = unsigned int]
The violated condition was: 
    partitioner.get() != nullptr
Additional information: 
    Unexpected initialization of Vector that does not set the TBB partitioner to a usable state.
--------------------------------------------------------

[drwells]

All of those tests pass on my laptop :( I am investigating.

[drwells]

@kronbichler I think that vector class is in better shape (it swaps partitioners correctly). I will take a closer look later.

[drwells]

I think I fixed the issues.

[masterleinad]

This passes the test suite for me locally. Let's merge.