Partition anisotropic lines onto one rank and make it the default - #2980

This PR splits off the parmetis improvements from #2931. It turns out that this parmetis improvement is 'almost' always better, and in the worst case performs equally well. It improves MPI performance for cases with inflation layers.

PARMETIS_ANISOTROPY_WEIGHT (default YES) weights the ParMETIS graph edges by cell aspect ratio, gives the edges that chain points into wall-normal lines the largest weight, and moves each line to the rank holding most of its points after partitioning. Lines are then not split between ranks, which keeps the block-Jacobi ILU and line-based agglomeration from seeing fragments of lines.

On stretched boundary-layer meshes this turned stalled or diverging parallel runs into converging ones (2D shear layer, 2D bump, 3D bump, flat plate with multigrid) and made iteration counts match the serial run more closely, with no case that converged without it failing with it. The load imbalance stays within 1.01-1.05 in 2D and up to about 1.3 on a large 3D mesh. The option has no effect on isotropic meshes or without ParMETIS; a warning is printed when it is set explicitly in a build without ParMETIS.

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