Add dn based output trigger

[pgrete]

PR Summary

What is says on the can (plus removed dead vtk code as I don't think we'll ever implement/reactivate it).

Still need

• doc
• changelog
• tests?

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• Change is breaking (API, behavior, ...)
  • Change is additionally added to CHANGELOG.md in the breaking section
  • PR is marked as breaking
  • Short summary API changes at the top of the PR (plus optionally with an automated update/fix script)
• CI has been triggered on Darwin for performance regression tests.
• Docs build
• (@lanl.gov employees) Update copyright on changed files

[BenWibking]

I can't tell whether the macOS CI failure is related or not:

The following tests FAILED:
	 68 - regression_test:boundary_exchange (Failed)
	 78 - regression_test:poisson (Failed)

It looks like some of the output files don't exist, so it might be.

missed failing CI

[pgrete]

I can't tell whether the macOS CI failure is related or not:

The following tests FAILED:
	 68 - regression_test:boundary_exchange (Failed)
	 78 - regression_test:poisson (Failed)

It looks like some of the output files don't exist, so it might be.

I think the updated handling of ensuring a last time based output is written might be broken (or not behaving as before any more).
I'm investigating.

[Yurlungur]

Assuming we can make tests pass, LGTM. 👍 for deleting dead code.

[pgrete]

The failed tests were related to a change in logic for setting dt and tlim by default as those tests did not include a temporal driver.
I now reverted that logic to the original state (i.e., dt=0 by default, which allows for the output being triggered).
I also added changelog and doc.
If all tests pass I'm now happy to merge.

[BenWibking]

Typo in docs. Otherwise LGTM

[Yurlungur]

Other than the minor docs typo LGTM