better way of specifying particle locations for screen icons

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This is a "code polishing" issue, mainly for my benefit.

I'd like to find a better (or at least more uniform) way of specifying the locations of particles for screen icons. For example, in GasPropertiesIconFactory createDiffusionScreenIcon, I currently have:

      // Particles, locations determined empirically, relative to centerX of divider, specified left to right
      const particle1Locations = [
        new Vector2( -400, 300 ), new Vector2( -600, 600 ), new Vector2( -340, 800 )
      ];
      const particle2Locations = [
        new Vector2( 400, 300 ), new Vector2( 660, 740 )
      ];

While this is in code that is unlikely to change and isn't related to anything else, it still feels a little fragile, and there was some trial-&-error ("empirically") required to arrive at the values.

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Think I'm going to leave this one "as is". The documentation is clear, and I don't think there's an obviously superior way to do this that doesn't involve a lot of work. So closing, and we'll see if it comes up in code review.