rename particlesGroupTandem to particlesGroup

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Related to dev test phetsims/tasks#821.

Skype:
[6/19/17, 12:40:56 PM] Steele Dalton: any reason you named it “particlesGroupTandem” instead of just “particlesGroup”? They show up in the Instance Proxies as “Particle Group Tandem_#”, but I would think it would look cleaner as “Particle Group_#”. Not sure I’ve seen the word “Tandem” in a tandem ID before.
[6/19/17, 12:44:37 PM] Chris Malley: no idea why it's named that. i recall collaborating on "group tandem" with SR. looking at other uses of createGroupTandem, I agree, should be called "particlesGroup".

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Done. @phet-steele please verify in master.

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FYI, here's what I see with instance-proxies wrapper:

Particles Group_0
molarity.molarityScreen.view.precipitateNode.particlesGroup_0: TNode

Particles Group_1
molarity.molarityScreen.view.precipitateNode.particlesGroup_1: TNode

...

[phet-steele]

Danke! Verified in http://www.colorado.edu/physics/phet/dev/html/molarity/1.3.0-phetiodev.3/wrappers/instance-proxies/instance-proxies.html?sim=molarity&relativeSimPath&phetioValidateTandems=false