Made it so clicking on already-annotated point hides annotation

Currently doesn't work with display="multiple"
Added "label" kwarg to set array of labels

A common case is where you want each point to be labelled with a value drawn from an array parallel to the XY array.  That is, you have a set of arrays X, Y, L, where X is the x-coordinates, Y is the y-coordinates, and L is some value associated with each point.  In this case you can just pass label=L to DataCursor and the corresponding label will be chosen from L based on X and Y.