rename chargedParticles to chargedParticleGroup

[pixelzoom]

Noted while working on https://github.com/phetsims/beers-law-lab/issues/244, where this sim was recommended as an exemplar for PhetioGroup. Same problem as https://github.com/phetsims/projectile-motion/issues/221.

In ChargesAndFieldsModel.js:

134  this.chargedParticles = new PhetioGroup( ( tandem, charge, initialPosition ) => {
141      tandem: tandem.createTandem( 'chargedParticleGroup' ),

The name this.chargedParticles is confusing. It's not charged particles at all, its a group that's used to instantiate charged particles. And it doesn't match the tandem name. Recommended to rename to this.chargedParticlesGroup.

Also worth noting:

• There are several other PhetioGroups created in this sim, and their field names have "Group" suffix (this.electricFieldSensorGroup, this.electricPotentialLineGroup,...)

• I find the pattern used in projectile-motion for PhetioGroup instantiation to be much nicer. That is, add a createGroup method to the class that needs to be instantiate via a PhetioGroup, rather than having new PhetioGroup throughout the code as is done in charges-and-fields.

[pixelzoom]

This has become a liability over in phetsims/tandem#180, where a PhetioGroup refactor is in progress. I can't just search for "Group.array". So I'll take care of this.

I confirmed with the iO team on Slack that this.chargedParticles should be renamed to this.chargedParticleGroup, to follow the convention of naming groups with "Group" suffix, and to follow the convention of using the same name for a field and its tandem.

Just to increase the level of difficulty, this sim has both {PhetioGroup} chargedParticles and {ObservableArray} chargedParticles.

[pixelzoom]

Done in the above commit. I also added assertions for the cases where there is {ObservableArray} chargedParticles.

@jonathanolson is the responsible dev, but commit history seems to indicate that @zepumph did the PhetioGroup instrumentation. I'll assign to both of them -- one of you please review and close.

[zepumph]

Changes look good to me! Thanks for the extra assertions for clarity. Closing