Several properties should be phetioReadOnly: true

[arouinfar]

For #76

These properties should be read-only, as there is no need for the client to directly set them.

• density.*Screen.model.gravityProperty
• density.*Screen.model.liquidMaterialProperty
• density.*Screen.model.pool.liquidYInterpolatedProperty
• density.*Screen.model.pool.liquidVolumeProperty

For each block:

• buoyancyForceInterpolatedProperty
• contactForceInterpolatedProperty
• gravityForceInterpolatedProperty
• sizeProperty
• userControlledProperty

[jonathanolson]

I believe these should all be handled with the above commit.

[arouinfar]

Looks good, thanks!