VQkit v1.1

QE charge-density-difference cube plotting update.

- Option [1] Charge-density difference is now functional for existing final QE .cube/.cub files.
- VQkit skips fragment selection when the final combined CDD cube already exists.
- Automatic detection prioritizes CDD/difference-named cube files.
- XY, XZ, YZ, and all-slice plotting are available.
- Each plotted plane produces PNG and numeric DAT output.
- Original cube remains unchanged for 3D visualization.
- 414 tests passed.
