PhET-iO: too many instrumented containers

[zepumph]

Over in phetsims/density-buoyancy-common#251 @samreid @AgustinVallejo and I would have liked to have been able to provide a tandem to ToggleNode just for its group items, and not instrument the ToggleNode class parent. This is not supported here, and not supported generally. This pattern often leaves a lot of extra and unnecessary instrumentation in published sims, like NodeIO instances.

We think this may be the biggest problem for common code that uses getGroupItemNodes because that pattern is prone to needing the group items instrumented at the cost of instrumenting the parent.

• This relates closely to Confusion around createNode( tandem ) and tandemName #822.
• This relates closely to https://github.com/phetsims/phet-io/issues/1989