When an ensemble is dropped on a histogram plot showing summary vector curves,
create a new curve per distinct vector and time step for the dropped ensemble,
matching the existing behavior for ensemble parameter curves.
* Add drag'n'drop support for ensemble summary vectors and ensembles into histogram plots
* Make sure histogram plot is shown and all update signals are connected when loading a project.
* Support ensemble parameter drag'n'drop from data sources into histogram plots.
* Show ensemble parameters in data sources tree.
* Add copy/paste of histogram curves
* Re-use all existing parameters in a plot when dragging a new ensemble into it
Kristian Bendiksen
JJ