NEW FEATURE: implement plotting a line when clicking on 2D plot

[Chilipp]

Summary

When clicking on a grid cell in a 2D plot of a 3D variable (or more), a line should
be visualized (or a 2D plot in case of 4D data)

Reason

It helps to investigate what happens in a grid cells

Detailed explanation

Ideally, one create a new formatoption that can take the following values

• False, nothing happens
• True, a line plot will be generated for 3D and a 2D plot for a 4D variable, when clicking on a grid cell
• str. Representing the dimension to use for a line plot for variables with more than 3D
• [str, str]. Representing the 2 dimensions to use for a 2D plot for variables with more than 4D

Examples

ncview has this useful feature

[Chilipp]

strongly related to psyplot/psy-view@4bec4c8, psyplot/psy-view#6 and the development of psy-view. Not sure yet, where this feature should live and how.