fix https://bugs.launchpad.net/yade/+bug/1368591 + remove a useless test
remove a temporary fix for https://bugs.launchpad.net/yade/+bug/923929, after better fix in https://github.com/yade/trunk/commit/4ea76ad6e47ac5074a389ad61712a0840e8560a5, thanks Anton
parallel removal of old interactions
replace LOG_ERROR by LOG_WARN for notifying deprecation of GravityEngine
improve falling back to 1-thread in parrallel collider (fix https://bugs.launchpad.net/yade/+bug/1368591)
fix missing brackets resulting in wrong permeability in periodic PFV