Operational updates

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A wide variety of updates have been made in the Sharrow PR, including core enhancements that change how ActivitySim work, and ancillary enhancements that add non-core features that are convenient for debugging and performance monitoring.

This PR isolates a limited number of non-core features so they can be add to version 1.1, allowing easier analysis and comparison against the sharrow changes coming for version 1.2:

• memory_profile option, which launches time-sampling memory monitoring in a separate subprocess.
• A more resilient resume_after that will fall back to restarting the model at the beginning if the selected checkpoint is not found (especially useful for setting the resume_after checkpoint as "_")
• An --ext option for the CLI interface for activitysim run which allows the user to give the location of an extension package of add-ons for a model; this can obviate the need for the simulate.py script in most of the examples with add-on packages.