Support plot with linked files

[zxdawn]

When we finish #24, we can modify s5p_lnox_plot_tracks.py to plot figures by linking order.

[zxdawn]

Well, we can split these two steps into different scripts:

• s5p_lnox_link.py: link swaths and calculate production efficiency
• s5p_lnox_plot_tracks.py: plot linked swaths

So, we will add modify the s5p_lnox_plot_tracks.py function to support plotting images in the linked order.