Cluster points that fall into the same grid cell.

Input parameters:

1. Pt → Points to group.
2. Cs → Side length of each grid cell. Points sharing a cell form a group.
3. Pl → Optional plane the grid is aligned to. When omitted, the best-fit plane through the points is used.

Output parameters:

1. Pt → Points in each group, one branch per group.
2. Ix → Indices of points in each group (relative to the input cloud), one branch per group.