This merge implements server.py for a visualization of the model.
For this implementation the mesa-provided CanvasHexGrid class needed to
be adjusted. Mesa's own function runs a visualization function on all
agents. For us it would be smarter to run the visualization function on
all grid locations, this is what CanvasHexGridMultiAgents does.
For now, concentrations are normalized using magic numbers. In the
future it might be smarter to somehow dynamically adjust these
normalizisation numbers to not threshold our visualization.
Initial implementation of step() accoriding to the rules of Schweitzer
et alii 1996.
A few neighbor fetching functions are now implemented in agent.py.
The MultiHexScalarFields implementation could be simplified as the ants
only drop their pheromones in advance() and not step(), thus they do not
interfere with each other.
We might need to look at the concentration decay for the scalar fields
again to correctly implement it. To me it is not clear whether we should
decrease the pheromone levels before the ants step() or after the ants
step() and before their advance()
