Construct staged integrators via arbitrary name

[pdmullen]

PR Summary

This MR alters the LowStorageIntegrator and ButcherIntegrator constructors such that they are capable of taking an arbitrary string. Previously, both constructors took as argument a ParameterInput *pin parameter and assigned an integrator name pin->GetOrAddString("parthenon/time", "integrator", "rk2"). This inhibited a downstream user from producing two unique instances of a StagedIntegrator that did not share the same stringed name. I attempted to ensure backwards compatibility of instantiating integrators via ParameterInput *pin; I did this by supporting an additional constructor (taking as argument ParameterInput *pin) that calls my newly added constructor (taking as argument std::string &name).

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
• CI has been triggered on Darwin for performance regression tests.
• Docs build
• (@lanl.gov employees) Update copyright on changed files

[Yurlungur]

Nice thanks for adding this. It's my bad this wasn't fixed when I was messing with integrators earlier.

[lroberts36]

LGTM