Feedback from user on XPS plugin

Today, we went through the XPS example from the docs (PA molecule) and also tried one example from the user's system (Al2O3 bulk).
Both cases agree well with the experimental data.

Some feedback on the improvement:

- [ ] explain the band gap correction for the insulator.
for example, Al2O3, the DFT calculated binding energy 77.8 eV, the experimental band gap ~ 7 eV, and the experimental spin-orbit splitting is 0.4 eV, so for the Al 2P 3/2,
```python
77.8 - 7/2 - 0.4/3 = 74.2 eV
```
experimental data is around 74.1 eV.
- [ ] When uploading the experimental data, explain the data format in more detail. e.g., csv file, and delete the title.
- [ ] support charge analysis
- [ ] Charge density with and without core-hole
- [ ] Bader charge with and without core-hole