Added utility functions for ensurance of pose transformation

These functions were formerly part of cram_moveit, but turned out to be non-moveit-specific. As they are used in other packages, they are moved to cl-tf2 to not introduce unnecessary dependencies towards cram_bridge (which contains cram_moveit).
Why do you need global var in ```cl-tf2```? This is supposed to be a library: Users of this library should make their own variables.