Allow Points to be Dragged

[DonIsaac]

What This PR Does

This PR allows users to click and drag any point on the screen. I would record a Loom to demonstrate dragging behavior but recording is bugged out for me right now. As points are dragged, both the SVG circle and the corresponding dot on the scatterplot are updated. Releasing the mouse invokes a DragHandler function, which is similar to a ClickHandler.

Additional Features

• New public methods for QuadTile:
  • visit_at_point(ix, callback): Finds a tile that contains a desired point and invokes a callback function. Uses a breadth-first search to check the targeted tile and it's children
  • find_point(ix): Finds a point with the target index. Consumes visit_at_point.
  • update_point(ix, new_point): Updates the data for the point at ix. Consumes visit_at_point.
• Add assert utility. Behaves similarly to node's assert, but does not require a polyfill