PR Summary

There is no reason to review this since I am not planning to merge this into Parthenon.

This example solves the equation $\partial_t \phi + \nabla \cdot (\vec{v} \phi) = - \phi / \tau$ using the IMEX RK integrators of Pareschi and Russo, where the flux term is treated explicitly and the source term is treated implicitly. It is really just an example of a possible way to write the task list for this type of integrator rather than a true test of the implementation, since this system doesn't really stress the integrator.

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