Using river flow to speed up movement across the adventure map

This is a continuation of map exploration that was started in the video on movement restrictions due to terrain, elevation and vegetation:
[Rumble] https://rumble.com/v3w3rnd-restricting-army-movement-with-terrain-elevation-and-vegetation.html

Commit: implement Flowing ability to reduce movement cost when using rivers for transportation (2023-11-26 10:17:47)