Change several properties to readOnly: false

[arouinfar]

For #244

This is a reversal of a request in #219 (sorry @zepumph!)

Several parameters are constant on early screens but adjustable on the Lab screen. We originally requested that these properties be made read-only, but this would prevent a client from adjusting the values using an API.

While @zepumph has mentioned that it's possible to make the properties ready only on a sub-set of screens, we thought it would be best to make these readOnly: false on all screens. This will allow the client greater control of customization.

@zepumph please make the following readOnly: false on ALL screens.

• [x] model.gravityProperty
• [x] model.projectileDiameterProperty
• [x] model.projectileDragCoefficientProperty
• [x] model.projectileMassProperty

[arouinfar]

@zepumph not sure if making these readOnly: false will automatically add a slider to the studio interface or not. If so, please disable it. We don't want to add a slider to studio to adjust these properties.

[zepumph]

Done. Note that I took out the readOnly, but added in phetioStudioControl: false, so there is still no slider to adjust in studio.

Anything else here?

[arouinfar]

Thanks @zepumph! Looks good in master.