Heatmap: add a GetCellIndexes() function to facilitate mouse interactivity

Suggested by @tonpimenta in #1787

It would be nice if heatmaps had a method to allow something like this:

```cs
Form1_MouseMoved(object sender, MouseEventArgs e)
{
    double? value = myHeatmap.GetValue(formsPlot1.GetMouseCoordinates());
    Console.WriteLine($"Value under mouse: {value}");
}
```