GAMG: ensure repeatability

Functionality to add/problem to solve

GAMG's agglomeration is sensitive to truncation errors in the face-weight. If the face-weights get calculated non-deterministic (e.g. offloading geometry calculation) this can dramatically alter the agglomeration from run to run.

Target audience

Codes using offloading.

Proposal

Add tolerance when doing pair-wise agglomeration.

What does success look like, and how can we measure that?

(more) repeatable runs