Check plotting in tutorial

[bifbof]

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.