While drilling down on overlapped tooltip, Currently processDrillDownForNodes function is called.
Modified to use the 'clickFn' if provided as part of initScatterChart config option which handles drilldown

Change-Id: Ia925c2952e47f43023286af892dad81c607e6960
Merge pull request #4 from nagakiran/featureDev_fixDrilldownOnTooltip

While drilling down on overlapped tooltip, Currently processDrillDownFor...