make the tiles on a selection a struct of info, not twinned lists (#143)

Currently, selections maintain a list of tiles and a list of how many matches there are in them that are guaranteed to be the same length by code.

It's better to have a list of records where each record contains *both* of these facts; we'll be using these much more to store sort information in a bit.
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| 🚀 | This description was created by 3c779e2  | 
|--------|--------|

refactor: use `SelectionTile` in `DataSelection` for tile management

### Summary:
Refactor `DataSelection` to use `SelectionTile` for managing tiles and match counts, simplifying data handling and preparing for future enhancements.

**Key points**:
- Refactor `DataSelection` to use `SelectionTile` for managing tiles and match counts.
- Introduce `SelectionTile` class in `src/selection.ts`.
- Replace separate `tiles` and `match_count` lists with a single list of `SelectionTile` objects.
- Update methods to use `SelectionTile`, including `moveCursorToPoint()`, `add_or_remove_points()`, and `wrapWithSelectionMetadata()`.
- Adjust iteration and selection logic for `SelectionTile`.
- Fix typo in comment: "untile" to "until".


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src/selection.ts

@@ -282,13 +282,21 @@ export class Bitmask {
     return result;
   }
 }
-export class DataSelection {
-  deeptable: Deeptable;
-  plot: Scatterplot;
+
+class SelectionTile {
+  public tile: Tile;
   // The match count is the number of matches **per tile**;
   // used to access numbers by index.
+  public matchCount?: number;
+  public indices?: number[];
 
-  match_count: number[] = [];
+  constructor(tile: Tile) {
+    this.tile = tile;
+  }
+}
+export class DataSelection {
+  deeptable: Deeptable;
+  plot: Scatterplot;
 
   /**
    * name: The name of the selection. This will be used as the colun
@@ -329,10 +337,10 @@ export class DataSelection {
    * The total number of points that have been evaluated for the selection.
    *
    * This is supplied because deepscatter doesn't evaluate functions on tiles
-   * untile they are loaded.
+   * until they are loaded.
    */
   evaluationSetSize: number = 0;
-  tiles: Tile[] = [];
+  tiles: SelectionTile[] = [];
 
   /**
    * Optionally, a user-defined for defining.
@@ -566,15 +574,12 @@ export class DataSelection {
     let currentOffset = 0;
     let positionInTile: number;
 
-    let current_tile_ix = 0;
-    for (const match_length of this.match_count) {
-      const tile = this.tiles[current_tile_ix];
+    for (const { tile, matchCount } of this.tiles) {
       if (tile.key === relevantTile.key) {
         positionInTile = rowNumber;
         break;
       }
-      current_tile_ix += 1;
-      currentOffset += match_length;
+      currentOffset += matchCount;
     }
 
     const column = relevantTile.record_batch.getChild(
@@ -634,7 +639,7 @@ export class DataSelection {
     });
 
     await selection.ready;
-    for (const tile of this.tiles) {
+    for (const { tile } of this.tiles) {
       // This one we actually apply. We'll see if that gets to be slow.
       await tile.get_column(newName);
     }
@@ -676,15 +681,14 @@ export class DataSelection {
     return async (tile: Tile) => {
       const array = await functionToApply(tile);
       await tile.populateManifest();
-      let matches = 0;
+      let matchCount = 0;
       for (let i = 0; i < tile.manifest.nPoints; i++) {
         if ((array['get'] && array['get'](i)) || array[i]) {
-          matches++;
+          matchCount++;
         }
       }
-      this.match_count.push(matches);
-      this.tiles.push(tile);
-      this.selectionSize += matches;
+      this.tiles.push({ tile, matchCount });
+      this.selectionSize += matchCount;
       this.evaluationSetSize += tile.manifest.nPoints;
       // DANGER! Possible race condition. Although the tile loaded
       // dispatches here, it may take a millisecond or two
@@ -721,14 +725,12 @@ export class DataSelection {
     }
     let currentOffset = 0;
     let relevantTile: Tile | undefined = undefined;
-    let current_tile_ix = 0;
-    for (const match_length of this.match_count) {
-      if (i < currentOffset + match_length) {
-        relevantTile = this.tiles[current_tile_ix];
+    for (const { tile, matchCount } of this.tiles) {
+      if (i < currentOffset + matchCount) {
+        relevantTile = tile;
         break;
       }
-      current_tile_ix += 1;
-      currentOffset += match_length;
+      currentOffset += matchCount;
     }
     if (relevantTile === undefined) {
       return undefined;
@@ -751,7 +753,7 @@ export class DataSelection {
 
   // Iterate over the points in raw order.
   *[Symbol.iterator]() {
-    for (const tile of this.tiles) {
+    for (const { tile } of this.tiles) {
       const column = tile.record_batch.getChild(this.name) as Vector<Bool>;
       for (let i = 0; i < column.length; i++) {
         if (column.get(i)) {