Add groupIdAccessor

[nickofthyme]

Is your feature request related to a problem? Please describe.
Allow groupIdAccessor props to specify the groupId of a series within a single spec.

Describe alternatives you've considered
The alternative is two separate specs that can each have it's own groupId. This causes labels to show only split accessor value and not the y accessor labels.

Additional context

<LineSeries
  id="line"
  xScaleType={ScaleType.Ordinal}
  yScaleType={ScaleType.Linear}
  xAccessor="x"
  yAccessors={['y1', 'y2']}
  splitSeriesAccessors={['z']}
  groupIdAccessor={({ yAccesor }) => yAccesor === 'y1' ? 'group1' : 'group2'}
  data={visData.rows}
/>

Then I can define two axes one for group1 and one for group2