Make is_prepared() more fine-grained

[roystgnr]

I'm still trying to figure out how to make this easy to use, but I'd like to at least test that what I've got so far still passes CI.

[lindsayad]

Does "do not merge" also mean "do not review"?

[moosebuild]

Job Coverage on 7c8c1d1 wanted to post the following:

Coverage

	ffa3a3	#3759 7c8c1d
	Total	Total	+/-	New
Rate	62.35%	62.36%	+0.02%	98.67%
Hits	68275	68328	+53	74
Misses	41235	41236	+1	1

Diff coverage report

Full coverage report

This comment will be updated on new commits.

[roystgnr]

Does "do not merge" also mean "do not review"?

It's already bad enough to need fixing, huh? ;-)

Nah, please jump in as you see fit. I've got more I'm wanting to add here, but if there's flaws in the foundation then it's probably better to fix them before I put more layers on top.

[lindsayad]

Just looking at the header this seems like a great foundation to me

[roystgnr]

This might actually be in shape to merge soon. My remaining concerns:

1. The additional internal testing I've added might trigger dbg and/or devel failures downstream, in tests of code that doesn't properly prepare a mesh before solving (so it'll fail the new assertions) but either doesn't rely on the unprepared data or just compares to a gold solution that mis-relied on it (so it's not already failing tests). I'll add every recipe I think might be affected to the CI here.

2. The additional internal testing I've added might be too expensive in dbg mode, where it tests preparation validity before every assembly. I think we can afford that (prepare_for_use() is much cheaper if we don't repartition, which we don't when checking validity). If we see CI timeouts then I'll try moving my testing from assembly methods to solve() methods. If we still see timeouts after that then I'll remove the intensive testing entirely for now.

[moosebuild]

Job Coverage, step Generate coverage on b0367ec wanted to post the following:

Coverage

Inconsistent report tags were found between the head and base reports.
This can happen when reports are missing from either the head or the base.

Inconsistent tags:
32bit-np2_threads2
Full coverage report

This comment will be updated on new commits.

[roystgnr]

I'm moving the "incredibly careful assertions" part of this PR to a new branch, https://github.com/roystgnr/libmesh/tree/mesh_preparation_assertions, which I'll put into a new PR for after this one goes in. That (plus the other PRs I've just factored out, plus one more bugfix PR that'll wait on #4394 because it requires a commit there) should get this PR down to just the functionality promised in the title, and should leave it in a ready-to-review state.