Uniformize vtkPlot API

Charts/Core/Testing/Cxx/TestChartDoubleColors.cxx

Charts/Core/Testing/Cxx/TestChartDoubleColorsOpaque.cxx

Charts/Core/Testing/Cxx/TestFunctionalBagPlot.cxx

Charts/Core/Testing/Cxx/TestHistogram2D.cxx

Charts/Core/Testing/Cxx/TestMultipleScalarsToColors.cxx

Charts/Core/vtkPiecewiseFunctionItem.cxx

Charts/Core/vtkPlot.cxx

Charts/Core/vtkPlot.h

Charts/Core/vtkPlotArea.cxx

Charts/Core/vtkPlotArea.h

Charts/Core/vtkPlotBar.cxx

Charts/Core/vtkPlotBar.h

Charts/Core/vtkPlotStacked.cxx

Charts/Core/vtkPlotStacked.h

Documentation/release/dev/uniformize-vtkplot-api.md