Group blocks together in the tree and use more general names

[arouinfar]

For #51

The block tandem names should be more general. Currently there are names like sameMassWood and sameDensitySmall but we can create more context by nesting things in the tree.

• [x] Intro Screen: follow the same pattern as density.compareScreen.model.blockSets, and call each block blockA and blockB. (We are adding A/B labels in #55.)
• [x] Explore Screen: follow the same pattern as density.introScreen.model.blocks and call each block blockA and blockB.
• [x] Shapes screen: currently called primaryMass and secondaryMass but "Mass" is a loaded term. Let's use objectA and objectB instead.

[jonathanolson]

Tandem structure should match this... however should the shapes screen have an "objects" tandem group? Also, the Shapes screen uses capsules now.

[arouinfar]

Thanks @jonathanolson. The checked items all look good to me.

however should the shapes screen have an "objects" tandem group? Also, the Shapes screen uses capsules now.

The capsule naming looks fine, but let's add a parent "objects" container, mirroring the structure of exploreScreen.model.blocks.

[samreid]

I added object levels for Shapes and also for the Applications screen:

This was straightforward and it seems all requests were addressed. A lot of unrelated changes appeared in the new API file. I guess I should check with @zepumph that those were intentional?? Feel free to close the isssue.

[zepumph]

buoyancy isn't stable, so it isn't currently getting updated very often. I have started reupdating density, buoyancy, and buoyancy:basics whenever I change the PhET-iO API of any of them. But this is just within that last 2 days. Can we all start doing that? My bound command is: grunt generate-phet-io-api --sims=buoyancy,density,buoyancy-basics