Use spatial field in the flocking model

[Datseris]

I propose that we change our flocking model in the main docs. At the moment the purpose of the model is to highlight our truly continuous space implementation. However, another advantage our continuous space has, which other ABM software does not have as far as I can tell, is its integration with spatial fields. I propose to add a spatial field on top of the space, something like a height map.

Birds feel a force from the height map that is equal to the gradient of the height map, as if birds prefer to fly over low ground than high ground. This would highlight the integration with move_agent! and the Euler timestep progress via update_vel!.