MueLu: Allow option to stop coarsening once aggregation stagnation is detected

[MalachiTimothyPhillips]

Enhancement

@trilinos/muelu

I'd like to have the option to stop the coarsening once we hit, e.g.:

******* WARNING *******
Aggregation stagnated. Please check your matrix and/or adjust your configuration file.Possible fixes:
  - reduce the maximum number of levels
  - enable repartitioning
  - increase the minimum coarse size.

It's also worth noting that I do have repartitioning enabled, so I'm not entirely sure why enabling repartitioning is listed as a possible fix.

Or, as per @csiefer2's idea, have a "coarse max size if MueLu won't coarsen any more" and use LU if below the threshold/crash if at or above the threshold.

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