1. Rename BexContainer to ForceContainer, O.bex to O.forces etc. Mostly backwards-compatible in python.
2. Add Shop::flipCell which changes shear without affecting interactions (by integral steps), demonstrated in scripts/test/periodic-shear.py (not yet fully tested, but doesn't crash ;-) ) [BROKEN due to recent VELGRAD changed]
3. Add InteractionsContainer::eraseNonReal