Added const accessor to data in realtime buffer

I'm not sure if this is a good solution to my problem at https://github.com/ros-controls/control_toolbox/pull/1

I need a way to get the data from a realtime_buffer when I only have a const copy of it.

Does this accomplish that? I know the realtime_buffer keeps a cached version of the data in case the mutex is currently being blocked, this function simply returns that version.

@ahendrix @wmeeusse @adolfo-rt

Thanks!