Meshblock Cost Function and Diagnostics

[Yurlungur]

PR Summary

Again part of the effort to get riot onto parthenon develop. Here I pull several features in from riot:

Memory Diagnostics Per Meshblock

Whenever variables or swarms are allocated or deallocated, memory used on a given meshblock is updated in a running total. Note this is only "interior" data. I also add the ability to look at the comm buffer pool, but this is a per-mesh quantity since the pool is shared. Not per block. I also didn't add the equivalent logic for boundary swarms, as I didn't fully understand the swarm machinery, and it's probably a negligible contribution. @brryan should probably take a look at the memory diagnostic logic for swarms in case I missed something.

I showcase this capability by modifying the sparse example to show memory footprint in bytes per cycle.

Manual load balancing

• I "made public" manual cost-based load balancing again. It's enabled in the input deck by setting parthenon/loadbalancing/balancer=manual.
• I also explicitly disable timers based load balancing, as this isn't fully threaded through the infrastructure anymore. I think it's not trivial to figure out what to do here, and I created an issue, Timers based load balancing #891 to discuss.
• I clarify what it means to use the manual vs default load balancing. Automatic load balancing is timers. Manual is the cost-based where downstream codes are expected to call the meshblock handles. Default is just meshblock count.

I also document both of the above new features.

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]

Resolves #576

[pdmullen]

LGTM

[lroberts36]

LGTM

[Yurlungur]

Athenapk folks: @pgrete @forrestglines do you want to take a look or should I just pull the trigger? I think the impact on athenapk is minimal.

[Yurlungur]

If I don't hear from AthenaPK folks by the end of tomorrow (Tuesday), I'll just pull the trigger. I don't think there's anything in here that should impact you.