Add plot_time()

[Rafnuss]

See #164 for background.

@damianooldoni

• Function name: time_plot() as suggested by @peterdesmet
• Axis: x-axis: timestamp, y-axis: grouping by any column of deployements (deployementID, locationID, or locationName) seems the only realsitic values
• feature: None implemented
• filtering: None implemented as suggested
• library: ggplot2
• plot style: no heatmap (yet?)