ASMStar: enable patch coloring

[pbrubeck]

Description

Enables ASMStarPC and ASMVankaPC to group patches by color (non-overlapping vertex/Vanka-stars) into a block diagonal matrix. Coloring is enabled through the petsc option -pc_star_use_coloring.

As a benchmark consider the Poison equation on a 256x256 mesh solved with one process:

Solver     | Degree | Iterations | Time     
-----------------------------------------
ColorASM   |      3 |          7 | 4.3655
PCPatch    |      3 |          7 | 13.104
TinyASM    |      3 |          7 | 7.9988
PetscASM   |      3 |          7 | 76.184

Here all solvers are mathematically equivalent, and all of them except TinyASM use Cholmod for the Cholesky factorization of the patch matrices.

Depends on https://gitlab.com/petsc/petsc/-/merge_requests/9069 (merged into release)

[rckirby]

This is a great idea -- it looks like user just adds a new option to the dictionary if we want to turn it on?

Probably we should have some minimal simple tests, such as parameterizing existing tests of ASMStarPC and ASMVankaPC to turn the coloring option off and on?

[ScottMacLachlan]

The changes in asm.py look quite reasonable to me. I'm not up enough on the netgen side to comment on that, though...

[JHopeCollins]

Really good that this is highlighted in the demos now.

I think this is very close, it's just very minor comments/queries.

Requested changes have been made.

[pbrubeck]

@JHopeCollins anything to add?

[JHopeCollins]

@JHopeCollins anything to add?

No this looks good to go now, thanks for addressing those last comments.

CI failures once again are unrelated docs link failures.