Check plotting in tutorial

Running the tutorial lead to some plotting result that could be improved.
Plotting positionfixes with `ti.plot` lead to a plot that seems to almost cut of some positionfixes.
`plot_module_split` by day works fine but the labelling is quite confusing as dates with the same day-in-the-month get the same label.
Changing these would make trackintel a bit more presentable.