opportunities for more use of validation options

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Related to #2 (code review);

• Are Validator validation options (valueType, validValues, etc...) utilized? These are supported in a number of core types like Emitter and Property. Is their presence or lack thereof properly documented?

There's good use of range for NumberProperty.

There is no use of valueType or isValidValue, and it looks like there are opportunities for use, specifically for Vector2Property and Property instances.

This issue code be deferred until PhET-iO instrumentation, when it will become more important.