Improvements to integrator output

[brbass]

Summary

• Adds an allReduceLoc utility so the controlling time step is printed once with its owning rank, rather than once per rank, when the time steps do not vary by proc.
• Adds verbose time step frequency. This allows the code to print out every N time steps, meaning the verbose time step can always be on but not flood the output.
• Allows periodic work frequencies in SpheralController to be callables, so they can change dynamically during a simulation.

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ToDo :

• Annotate RELEASE_NOTES.md with notable changes.
• Create LLNLSpheral PR pointing at this branch. (PR#)
• LLNLSpheral PR has passed all tests.