Add a direction of flow indicator to `map_xs`

[mpdougherty]

When zoomed in tight on a cross section, one has to remember from the reach overview map, what is the direction of flow. A "direction of flow indicator" would be helpful to orient the reader.

[mpdougherty]

The traditional approach would be to add an arrow to the flowline pointing in the direction of flow. However, tmap::tm_line does not support adding arrows to lines.

Since the XS map is at such a fine scale, another approach could be to simply draw a small "director of flow indicator" in the upper left corner of the map. The flowline_points could be added as a new parameter and the upper and lower point can be identified from the map extent and used to calculate a direction vector used to draw an arrow.