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()
