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

I have checked that my code follows the Octopus coding standards
I have added tests for all the new features added in this request.