[rans][fluxvars] adapt coefficient averaging with boundary check

Merged Ned Coltman requested to merge bugfix/rans_fluxvarsboundaries into master

I've been getting a segfault for rans models run on 3D grids cut with subgrid. Address sanitizer pointed me here, and it seems like the boundary check wasn't sufficient. This should fix that. All tests still pass on my machine.

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