ChartMeasure.outputPlot() - Remove var-args measures and charts param

As mentioned here - https://github.com/graphstream/gs-core/issues/121

Removing var-args option for charts and measures as it's not supported anyway and makes it's use a bit confusing. In addition, you could simply call outputPlot() multiple times, with different charts (unless I'm misunderstanding it's purpose).

Also, minor README update.