Check PDBQT CRYST1 reading

[IAlibay]

Follow up from #3132

In the PDB standard a CRYST1 from a non-crystallographic system is set to [1,1,1,90,90,90], in those cases we set dimensions to None. For the PDBQT reader we just read whatever is in CRYST1, we should check if the same behaviour as the PDB reader needs to be follow here. See: https://github.com/MDAnalysis/mdanalysis/pull/3132/files#r628898514

Additional task:

• Add coverage to convert_pos_from_native, see: https://github.com/MDAnalysis/mdanalysis/pull/3132/files#r629699881

[IAlibay]

I can't find any docs on this, I'm going to move this to 2.1.0.

[orbeckst]

Can we make this a GSoC issue if we agree that we want to same behavior as for PDB?

[IAlibay]

Possibly? I'm not sure how obscure the standard is, and if it's meant to follow pdbv3.. I'm not sure if it would be a bit too much to ask someone to dig into this.