FIX: change class members and functions from private to protected
During migration to kinetic some class members and functions moved from protected to private. This breaks the API. E.g, the loop_rate_ variable has to be set from derived classes. This can only be done by setting it to protected instead of private or either creating a SET function.
This branch moves back to protected those class members to keep compatibility.
Tested with Ubuntu 16.04 and ROS Kinetic.