Add a vector wave equation example

[lroberts36]

PR Summary

This PR adds a vector wave equation to the fine_field example. That is, we solve the equations

\partial_t \vec{C} + \nabla \times \vec{D} = 0

\partial_t \vec{D} - \nabla \times \vec{C} = 0

\nabla \cdot \vec{C} = 0

\nabla \cdot \vec{D} = 0

using an upwind constrained transport scheme (currently just using piecewise constant reconstruction). The face centered fields are evolved and prolongation is done using the scheme of Toth & Roe.

TODO:

• [x] Actually initialize $\vec{C}$ and $\vec{D}$ to something
• [x] Include Toth & Roe prolongation so that divergence constraint is preserved in AMR

PR Checklist

• [x] Code passes cpplint
• [ ] New features are documented.
• [ ] Adds a test for any bugs fixed. Adds tests for new features.
• [x] Code is formatted
• [x] 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)
• [x] CI has been triggered on Darwin for performance regression tests.
• [ ] Docs build
• [ ] (@lanl.gov employees) Update copyright on changed files

[Yurlungur]

Include Toth & Roe prolongation so that divergence constraint is preserved in AMR

Is this needed if using edge fields?

[lroberts36]

Include Toth & Roe prolongation so that divergence constraint is preserved in AMR

Is this needed if using edge fields?

@Yurlungur: No, it shouldn't be. I had started writing it using face fields though.

[lroberts36]

@par-hermes format