print_helper using 3D vector argument

[miaoyinb]

closes #3380

[miaoyinb]

@GiudGiud FYI

[jwpeterson]

FYI @miaoyinb I sent you an invite to join a group that has its PRs tested automatically. Note that some of the testing may fail currently due to a change that is present in libmesh upstream that is not compatible with MOOSE, we're hopeful that will be updated soon.

[moosebuild]

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[miaoyinb]

FYI @miaoyinb I sent you an invite to join a group that has its PRs tested automatically. Note that some of the testing may fail currently due to a change that is present in libmesh upstream that is not compatible with MOOSE, we're hopeful that will be updated soon.

Got it. Thank you so much.

[roystgnr]

I dislike that we're flattening the output here rather than using some kind of delimiter between different sub (and subsub) vectors ... but we're already flattening the vector<vector<P>> case, so at least we're consistent.

[roystgnr]

The Test MOOSE failure is just "Test MOOSE python" again.