MultiScatter/Scatter: refactor logic for providing fill to <Symbol />

[GabeMedrash]

Currently, the only way to reliably provide the same fill to an entire <Scatter /> is to provide a dataAccessor.fill. While <MultiScatter /> exposes fieldAccessors.color, relying on it depends on const fillValue = propResolver(datum, dataAccessors.fill || dataAccessors.x); resolving to a non-finite value. That's wonky!

Providing fieldAccessor.color should reliably determine fill for an entire <Scatter />.

[GabeMedrash]

Notes:

• default MultiScatter.defaultProps.colorScale to _.identity
• use case of inconsistently applying color scale
• whether or not Scatter.props.fill takes precedence in Scatter::render