factor out IO type constants

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In light of how IO types increase memory footprint (see phetsims/tandem#71), consider factoring out these sim-specific phetioTypes values:

    // related to Quadratic
    PropertyIO( NullableIO( QuadraticIO ) )
    DerivedPropertyIO( QuadraticIO )
    DerivedPropertyIO( NullableIO( QuadraticIO ) ) 

    // related to Vector2
    PropertyIO( Vector2IO )
    DerivedPropertyIO( Vector2IO )
    DerivedPropertyIO( NullableIO( Vector2IO ) )

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If this turns out to be a significant memory savings, the same should be done in sim-specific code for other instrumented sims.

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Mentioning @samreid, since we had discussed the potential benefit of this on Slack, in relation to phetsims/tandem#71.

I tried factoring out constants for the phetioType values identified in #105 (comment). Here are the heap snapshots on Chrome 70.0.3538.77 + macOS 10.11.6, ?ea&brand=phet&fuzz, before and after factoring out these constants.

Time (minutes)	Before	After
0	44.2 MB	44.0 MB
2	51.5 MB	51.9 MB

Since there is no obvious memory reduction, I'm not going to commit the changes. I think it's clearer in this case to have IO type instances next to the phetioType keys.

For posterity, here's what I added to GQConstants, but did not commit:

    // phetioType values related to Quadratic, see https://github.com/phetsims/graphing-quadratics/issues/105
    PROPERTY_IO_NULLABLE_IO_QUADRATIC_IO: PropertyIO( NullableIO( QuadraticIO ) ),
    DERIVED_PROPERTY_IO_QUADRATIC_IO: DerivedPropertyIO( QuadraticIO ),
    DERIVED_PROPERTY_IO_NULLABLE_IO_QUADRATIC_IO: DerivedPropertyIO( NullableIO( QuadraticIO ) ),

    // phetioType values related to Vector2, see https://github.com/phetsims/graphing-quadratics/issues/105
    PROPERTY_IO_VECTOR2_IO: PropertyIO( Vector2IO ),
    DERIVED_PROPERTY_IO_VECTOR2_IO: DerivedPropertyIO( Vector2IO ),
    DERIVED_PROPERTY_IO_NULLABLE_IO_VECTOR2_IO: DerivedPropertyIO( NullableIO( Vector2IO ) )