Merge branch 'barchart' into 'master'

Barchart add-on for plotter gui-plugin

The plotter now has a dropdown menu to change the visualization style from curves to bars. 
The bars are dynamically scaled and added and receive the data in the same time as the curves.
The colors of bars and curves match.
Adding more styles in the future is easily possible due to stacked layout and singular distribution of data.

See merge request !3