VQkit v1.1 - QE CDD cube direct plotting

Charge-density difference option [1] now detects an already-generated final QE Gaussian cube file and plots it directly.

Behavior:
- Prefer filenames containing CDD, difference, charge_diff, delta_rho, etc.
- If exactly one final CDD cube is found, no fragment questions are asked.
- If several plausible CDD cubes are present, VQkit asks only which final cube to use.
- Supports XY, XZ, YZ, or all center slices.
- Outputs VQkit_QE_CDD_XY/XZ/YZ.png and matching .dat files.
- The original cube is preserved unchanged for 3D isosurfaces in VESTA, VMD, or XCrySDen.
