Documentation updates
Add melted option for performance
Fix curves plotting with different stats
Add bar plot performance example to vignette
Add melted example in vignette
Remove groups from variant no 3 (BLP)
Change groupnames behaviour
Implement groupnames for Zcurves

* Refactor groupname extraction to utilities
Refactor zcurves to ZCurvesDataFrame and ZGroupCurvesDataFrame
Update docs
Refactor group plotting

Works now only for groups
Performance curves plotting finsihed