PhET-iO Customization for Density Mystery Blocks

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In Density 1.1, there was an option to Mass.ts called adjustableMaterial which allowed each materials to have a custom color/density/material selection. Since #256 this is no longer implemented, and we need to add support for this.

This issue may not need much work, but we want to have a ticket to make sure.

This issue is on hold until:

• Preallocate all Material instances and make the density mutable #256 is complete and denisty mystery screen materials are pre allocated
• Manage material colors now that materials are mutable #268 supports customizing colors (probably) for these custom materials.

[zepumph]

Off hold now, commits from the TODOs will come to this issue.

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• See ColorProfile is incomplete and also contains unused colors #265 Uninstrument color Properties in density? I think we should ask @arouinfar. They aren't important now (and never were?).

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• the non-random sets still need a way to customize color.

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Everything else is in side issues (see above and here).