Added magnetic data cursor

[siriuspal]

Added "Magnetic" Data Cursor keyword argument.

For artists with x and y attributes, such as line chart, magnetic will only create data cursor on a data point in chart. By default the library creates cursors on interpolated lines between data points. This can be inconvenient in some cases where exact values of data points are needed. If magnetic=True, cursor will be created to the data points closest to the mouse click by linear interpolation.

Tested the changes with all examples in the repository. For artists which do not have x and y attributes (such as scatterplot, image) magnetic will not have any effect.