Use JSON for the calibration file
This is a reduced idea from !980 (closed). It just ports what we already do for calibration so that it uses JSON. The issues addressed in !980 (closed) like proposing a more general calibration file and passing calibration data to SLAM systems will be postponed.
It also updates the slam_tracker.hpp
to allow "special features" to be implemented in the external SLAM systems, although it does not use any for now.
Edited by Mateo de Mayo