Refactor to allow non-quadtree arrow datasets about deepscatter HOT 2 CLOSED

Turns out the direction this is going is to pull back a little bit from the "Quadtree is all you need" approach I've been using and to let some of the methods that currently live in the root quad tile exist instead in a new abstract class called Dataset that can be extended to run on quadtrees but also arrow tables, duckdb databases, arquero tables, whatever.

This will also formalize the idea that tiles are things that can generate Arrow record batches. The number of methods for each one of these isn't extremely large, although they'll mostly new subclasses of Tile as well (to handle things like descendants.)

export abstract class Dataset<T extends Tile> {
  abstract root_tile : T;
//  public mutations : 
  public max_ix : number = -1;
  protected plot : Scatterplot;
  protected _tileworkers: TileWorker[] = [];
  abstract ready : Promise<void>;
  constructor(plot : Scatterplot) {
    this.plot = plot;

from deepscatter.

This is merged.

from deepscatter.