Provide element index with label_formatter

[upsj]

label_formatter only provides the name of the plot and the position of the point,
but that makes identifying the exact data point unnecessarily complicated,
when the index of the data point is available internally.

To change that, I wanted to add some more generic functionality, which either requires an interface break or adding a new function - I decided for the latter.

I added a new enumerated_label_formatter interface that introduces three improvements:

1. Instead of passing an empty string "" for the label when no data point is selected, we pass an Option
2. Instead of returning a String, we return an Option, which allows hiding the label when we don't need it, e.g. when not hovering over a data point.
3. Instead of taking just the position and plot name, we take the position, data point index and plot name.

• I have followed the instructions in the PR template

[upsj]

There is perhaps a longer discussion to be had about the fact that the label_formatter has two purposes - providing a label in empty space, or near data points. Maybe an enum could be the better input here? (The naming is bad, but I also didn't think about it much)

enum HoverPosition {
    NearDataPoint {
        hover_position: Point,
        plot_name: &str,
        data_position: Point,
        data_index: usize
    },
    Elsewhere { hover_position: Point }
}

[upsj]

I replaced the Option usage by an enum, that makes the interface a bit more descriptive, albeit maybe also more verbose?