VQkit v1.1 — Quantum ESPRESSO ELF update

- Detects existing ELF .cube/.cub files automatically.
- Uses an existing final QE ELF cube directly; no recalculation is required.
- If no ELF cube is found in a QE directory, writes VQkit_QE_ELF_pp.in with plot_num = 8.
- Supports XY, XZ, YZ, all center slices, and custom fractional slice positions.
- Uses a fixed dimensionless ELF plotting range of 0.0 to 1.0.
- Adds publication-style filled contours, ELF contour guides, atom markers, and element labels.
- Exports 400 dpi PNG plots plus numerical .dat slices.
- Retains the original cube unchanged for VESTA/VMD/XCrySDen.
