Implementation of multiprocessing for tracks loading

through the "workers" setting in the config file (the repatriation of the gpx dom, which takes a long time to be serialized / deserialized is delayed) -> loading time divided by up to 4 on a 6-cores cpu
with automatic determination of the number of physical cores if "workers" is "true" or "*"
addition of color for messages related to track loading in order to distinguich more easily errors
possibility to interrupt the loading of the tracks and leave the script by pressing "S"