Simple grassing system with reproduction and grass/search behaviour

@reichold @nolzen Another small model...

Simple grassing with metabolism, reproduction and two different behaviours.

I think this examples is well digestible and demonstrates some important ECS features:

• Resources
• Systems operating on entities, systems that do not
• Adding/removing entities
• Adding/removing components from entities
• Different types/compositions of entities

The mayor flaw I see is that components IsSearching and IsGrassing could be replaced by a simple boolean. However, maybe this is not too bad as is shows how different behaviour can be implemented in different systems. Also, it would easily enable to add more behaviours. What do you think?

I think I will try to structure this more in line with ODD to show that ODD and ECS play together nicely.