On a pivoted table, the group tree column doesn't remember resizes

## Bug Report

### Steps to Reproduce:
1. Go to the perspective [homepage](https://perspective.finos.org/)
2. Change the perspective viewer plugin to the datagrid
3. Resize the group tree column (first column)
4. Click on any of the other column headers to sort

### Expected Result:
The column resize to be maintained after the sort

### Actual Result:
The column resize is lost after the sort