Avoid template specialisation tricksiness.
Merge remote-tracking branch 'felix/master'
Track low/high.
Introduce EffectivelyZero.
Use EffectivelyZero where needed.
Fix warning.
Prune the tree if first value is 0.
Use typedefs to make things less messy.
Yet another level of virtual.
Another level.
Make the rest virtual.
Add benchmarking and optional caching for SelfContract2edTensor.
Fix caching bug.
Clean up output.
About 10% speedup.
Another 15% speedup.
Another 15-20%.
Another 20%.
Pedantry.
Small build fix.