Add dataSources/variables #23
Description
A Plot (and maybe a Session/tab) should have a set of dataSources that the user could use to build or modify plots.
This dataSources would be available to the plot editor in the GUI.
Probably sisl objects should also have dataSources. This would allow building plots using a modified version of plotly express ("sisl express") as easily as sx.histogram(geom, x="z", color="neighs")