Improve docs/code of CSVTrajectoryLoader regarding lat/lon

[Lucas May Petry]

The docs of the CSVTrajectoryLoader class are not very clear about how the latitude and longitude attributes of datasets are loaded. Maybe we need to change the code/docs to make it more user friendly.

[Antonio Carlos Falcão Petri]

I would like to understand why TrajectoryData needs the combined feature lat_lon. As it seems to me, calculating lat_lon in CSVTrajectoryLoader#load_tids only presents a performance bottleneck (even considering parallelization) since it needs to iterate over each trajectory point in a Python loop. Also, it forces each point data to be a list, instead of relying solely on np.array.