Stamina system at unit level

[Sundae2608]

At the moment, the impact of troops on each other is simply hard-coded, with no consideration of terrain. By implementing the system using actual unit mass, we can potentially:

• Create a much more realistic charge impact. For example, cavalry charging downhill should have much more impact compared to cavalry charging uphill.
• Realistically simulate the fatigue of the soldiers, based on the actual amount of energy they spent to perform their actions. For example, units moving uphill should receive far bigger penalty on stamina, compared to units moving down-hill.

[Sundae2608]

Rename the task to better reflect the task and the design.