VQkit v1.1 — Publication-style CDD plotting upgrade

For an existing final Quantum ESPRESSO CDD cube, VQkit now provides:
- smooth filled-contour plotting instead of pixel-block rendering;
- symmetric accumulation/depletion scale around Delta rho = 0;
- QE cube-density conversion from e/bohr^3 to e/Angstrom^3;
- atom markers and element labels for atoms close to the selected slice;
- XY, XZ, YZ center slices;
- custom slice positions (fractional 0–1 along the plane normal);
- planar-averaged Delta rho(z);
- 400 dpi PNG output;
- numeric DAT export in publication units;
- original cube retained unchanged for VESTA/VMD/XCrySDen 3D isosurfaces.

Red = charge accumulation; blue = charge depletion.
