Integrate flow field pathfinder into game simulation

[heinezen]

Makes the pathfinder usable from the game simulation.

Depends on https://github.com/SFTtech/openage/pull/1648, https://github.com/SFTtech/openage/pull/1647

Introduces several additions and optimizations to the work started in https://github.com/SFTtech/openage/pull/1620 :

• Flow field documentation
• Gamestate
  • Map object for accessing pathfinder
  • use pathfinder in move system
  • new nyan API bindings for Terrain objects
• Pathfinder
  • Massive performance increase for flow field calculation
  • LOS propagation across sectors
  • Flow field caching
  • Fixed several bugs