We are using RTKLIB for RTK which is written in C++:
I’ve not tried to inject data in to the calculation. I would probably use a separate kalman filter if I was going to do that. This page talks about how it would be done in ROS, but serves as a good overview generally:
http://docs.ros.org/en/melodic/api/robot_localization/html/integrating_gps.html
I am also evaluating this as a possible GPS+IMU solution: