memory test for brand=phet-io (1.1 branch)

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This is one of the "standard GitHub issues" required by the code-review checklist.

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On hold until the memory leaks identified in brand=phet are fixed, see #189 (comment).

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Some big iO memory leaks were identified for brand=phet, see #189 (comment). So it's time to see what the state of brand=phet-io is.

I'll be using the same test procedure described in #189 (comment), but running with
?brand=phet-io&phetioStandalone&secondsPerGeneration=0.25

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Test results with local unbuilt, natural-selection sha e32a316, with the patches from phetsims/tandem#203 and phetsims/axon#328 applied.

Test configuration: 2019 MacBook Pro (MacBookPro16,1), macOS 10.15.6, Chrome 84.0.4147.135

Generation	Heap Size (MB)
0	27.9
100	57.8
200	60.8
300	60.8
400	58.7
500	61.8
600	59.0
700	58.0
800	60.6
900	58.6
1000	59.7
0	30.4

This looks nice. With a stable population, the footprint is immediately stable at ~60MB. It was 45BM for brand=phet in #189 (comment), so there's not a huge increase for brand=phet-io. And the Generation 0 population is fairly consistent.

Work is done here. On hold until phetsims/tandem#203 and phetsims/axon#328 are addressed. Then I will sanity check with one more test.

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After fixes for phetsims/tandem#203 and phetsims/axon#328 were pushed...

Testing results on 2019 MacBook Pro, macOS 10.15.6, Chrome 85.0.4183.83 (Incognito), with https://phet-dev.colorado.edu/html/natural-selection/1.1.0-dev.17/phet-io/natural-selection_all_phet-io.html?postMessageOnError&phetioStandalone&secondsPerGeneration=0.25

Generation	Heap Size (MB)
0	26.0
100	55.1
200	55.5
300	58.4
400	55.0
500	55.3
600	55.7
700	57.6
800	57.7
900	55.9
1000	56.0
0	28.4

This looks great. Stable memory with stable population, consistent memory for Generation 0. And ~24% more memory than brand=phet (~45MB vs ~56MB), see #189 (comment).

Closing.