For of instead of in fix (#166)

<!-- ELLIPSIS_HIDDEN -->



> [!IMPORTANT]
> Fix loop iteration in `domain()` function of `Deeptable` class from `for...in` to `for...of`.
> 
>   - **Behavior**:
>     - Fix loop iteration in `domain()` function of `Deeptable` class from `for...in` to `for...of` for iterating over `subfield` array.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis" src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=nomic-ai%2Fdeepscatter&utm_source=github&utm_medium=referral)<sup> for c33efd4. It will automatically update as commits are pushed.</sup>

<!-- ELLIPSIS_HIDDEN -->

src/Deeptable.ts

@@ -364,7 +364,7 @@ export class Deeptable {
 
     // First -- look at the schema metadata.
     let dim = this._schema?.fields.find((d) => d.name === columnName);
-    for (const sub in subfield) {
+    for (const sub of subfield) {
       if (dim === undefined) {
         continue;
       }