Flow field pathfinding

[Seb-stian]

Implement flow fields for entity pathfinding. There are some cases, where A* is not the best choice:

• If the graph that needs to be traversed is dense (A common example would be a grid-based graph) ✔️
• If you have hundreds of units attempting to path to the same location ✔️
• If the environment is highly dynamic ✔️
• If there is a constant need to change the position of units ✔️
  (source)

Flow fields can be of great benefit for us. Some things to consider:

• Implementation may be difficult to get right (eg. caching fields)
• We may want to toggle between A* and Flow fields depending on distances and entity count
• Different entities may need somewhat different vector fields (eg. zombie x spider)

For now, A* should do just fine, but I would like to see this implemented in the future.