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).
Okay, I just introduced the dependency as both, `cl_tf` and `cl_tf2` reside in the same ROS package (`roslisp_common`). The global var is gone now and all functions have their respective parameters for an instance of type `cl-tf2:buffer-client`.