Cleanup of global.model.materials

[arouinfar]

Related to phetsims/buoyancy#51.

I reviewed the updated materials for #250, and identified some needed cleanup.

• Move sand to materials.solids
• Rename mysteryY > materialY.
• All non-custom, non-mystery materials should have a read-only density.
• Uninstrument all materials that aren't user-selectable:
  • air
  • boatHull
  • diamond
  • glass
  • human
  • lead
  • titanium
• Materials should only appear in the trees of relevant sims. (No non-water fluids in Density, for example.)

Over to @AgustinVallejo. Please let me know if you have any questions.

[AgustinVallejo]

I believe this is done, back to @arouinfar to review.

[samreid]

Uninstrumenting the boat body caused the CT error which is also tracked in #301. I'll close that issue and address it here

[arouinfar]

Overall, the changes look good. If I'm not mistaken, @samreid reinstrumented the boat's material, which in retrospect, seems like useful information for the clients. Thanks!

@samreid @AgustinVallejo can you please rename boatBody to boatHull? It was previously boatHull, which is the more appropriate terminology.

[AgustinVallejo]

Renamed the tandem, closing. Also, thanks @samreid for fixing the CT issue! Didn't catch that