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