Test Mixed models within unit, Pathfinding

added first unit with mixed appearance within the unit. It is the horse archer unit, and has a handful of heads, bodies, and legs that can be swapped out. It also swaps between a bow and sword for shooting/melee. This is so far a kind of prototype of how to do this, and redoing existing units with mixed models like this will be a decent amount of work. Also swapped out a lot of the pathfinding, including the type of navmesh graph, and added some movement penalty to pathing through units in combat so the ai is more likely to take units around that, and fixed up flank attacks in some ways, and various other bug fixes and general improvements.