Adaptive integrator

[srichers]

Summary

Added adaptive timestepping functionality to RKIntegrator with a Dormand-Prince butcher tableau

Additional background

I previously tried using adaptive timestepping with SUNDIALS, but it was not compatible with particle data structures. I added logic in the location suggested by code comments to implement adaptive timestepping. This also required a bit more information about the Integrator state to be available. I did not provide a function for computing a normalized error that would treat both relative and absolute tolerances, since the absolute tolerance needs to be normalized to a potentially unitful quantity in a nontrivial way. Rather, I provide a setter function so the user can define the error function. My example error function for the Emu code is at https://github.com/AMReX-Astro/Emu/blob/adaptive_integrator/Source/main.cpp#L306. RKIntegrator should not be affected if the DOPRI tableau is not used.

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate