Prism::opposite_side()

[roystgnr]

This isn't well-defined on every side, but it is on two of them.

Thanks to @zachmprince for pointing out that we might be missing this. The "march away from a boundary" code I've used on quad and hex meshes in the past would also be reasonable on the common "prism boundary layer extruded from triangular surface mesh" meshes.

[moosebuild]

Job Coverage, step Generate coverage on d3fc441 wanted to post the following:

Coverage

	b213b1	#4195 d3fc44
	Total	Total	+/-	New
Rate	63.70%	63.70%	-0.00%	0.00%
Hits	75766	75766	-	0
Misses	43177	43180	+3	3

Diff coverage report

Full coverage report

Warnings

• New new line coverage rate 0.00% is less than the suggested 90.0%

This comment will be updated on new commits.