TRI7 center node adjustment and TRI6/7 degeneracy checks after midpoint relocation

[miaoyinb]

closes #4480

[miaoyinb]

As this PR moves TRI7 center node, MOOSE's meshgenerators/xy_delaunay_generator.xydelaunay/quadratic_tri7 test needs to be regolded.

[moosebuild]

Job Coverage, step Generate coverage on 4659afd wanted to post the following:

Coverage

	862243	#4481 4659af
	Total	Total	+/-	New
Rate	65.60%	64.84%	-0.76%	100.00%
Hits	78626	77754	-872	58
Misses	41235	42165	+930	0

Diff coverage report

Full coverage report

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[roystgnr]

As a general rule "midpoint goes at the centroid" isn't always right, since you can do some clever things with boundary layers by using slightly-off-center higher order nodes in the grading, but in this context I think this is exactly the right thing to do.

If you don't want to create a big "have to get the submodule updated to regold" vs "can't merge the PR without breaking a ton of recipes" chicken-and-egg problem, the general way to do that is to add new behavior as an option before we change behavior by default. Default the new behavior to "off", then after MOOSE is updated turn it on there and regold, then change the default to "on".

[miaoyinb]

Good point, I will make it optional.

[moosebuild]

Job Distributed make check sweep (odd) on 4659afd : invalidated by @miaoyinb