Example structure SiO2 has extra tags

[superstar54]

Describe the bug
Feedback from a PSI user:
I tried calculating XAS spectra for SiO2 (from the example structures). The SiO2 structure gives me the error < ValueError: Error occurred validating port 'inputs.pw': The pseudos specified and structure kinds do not match: {'O', 'O1', 'Si', 'X'} vs {'O', 'Si', 'X'} >

Additional context
We should remove the tag of the SiO2 structure.

[PNOGillespie]

Hi @superstar54. Just wanted to add a few things here:

1. I would be interested to try re-creating the error, since I'm fairly sure something like this should be fixed by (XAS: Enable Correct Parsing of Hubbard and Magnetic Data aiidateam/aiida-quantumespresso#969) but I'd like to make sure this is the case.
2. Why does the structure have this special marker for oxygen anyway? I'm just curious about that.

SiO₂ is also used in the XSpectra literature (section A of the 2009 paper), and their past tutorials, as an example for Si and O K-edge calculations. The structure used there is hexagonal, rather than the one currently in the examples which is tetragonal, so I could provide a replacement for the structure already in the AiiDALab-QE examples if we want to do that.

Let me know your thoughts on that.

[superstar54]

The tag is removed in #518.

The structure used there is hexagonal, rather than the one currently in the examples which is tetragonal, so I could provide a replacement for the structure already in the AiiDALab-QE examples if we want to do that.

Thanks @PNOGillespie. Some structures in the example are incorrect, and @cpignedoli is working on it.