Roadmap: New default algorithms for 17.0

In order to improve user experience, we should change the following default algorithms:

• Exponential method should be Chebyshev instead of Taylor: much more stable (!2741)
• Eigensolver should be Chebyshev filtering, not CG (works much better, and on GPUs) (!2742 (merged))
• LCAO should be LCAO Alternative (works on GPUs and is faster). (!2843 (closed))
• Change mixing strategy for magnetic systems (!2820 (merged))

Less important but still to be changed:

• ACE operator for hybrids/HF (!2738 (merged))
• Rename LCAOAlternative (!2740 (merged))
• Deactivate pseudopotential filtering for HGH (!2661 (merged))
• Link EigensolverTolerance and ConvRelDens (!2799 (merged), !2844 (merged) )
• Deactivate the Chebyshev oracle for spinors (!2830 (merged))

Change of behavior:

• Remove default time-step (!2739 (merged))
• Add fully relativistic pseudodojo PBE pseudo (and test)