Update experimental status for several features
Description
Removed the experimental status from the following features:
- Non-orthogonal unit cells
- PseudopotentialSet = sg15
- SymmetrizeDensity
- Time propagation for periodic systems
- TDOutput = total_current
News snippet
Removed the experimental status from several features that are now considered well-tested.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.