Changed the ant pheromone drop.

- Now ants will drop a pheromone quantity that is relative to the path
length.
- This is not a global mechanism, paths are not compared.
- You can set this behavior using a gamma paramter. If gamma is 0 the
ant drops a constant quantity of pheromone on its path. If gamma is 1
the quantity of pheromone is a drop / pathLength, if gamma is > 1 the
quantity is drop / pow(pathLength,gamma) (you can also do strange
things with gamma > 0 and < 1)
- The ants now remember and count their current path length.
- Added minPh, maxPh, phDrop and gamma as parameters that can be read
from the graph.
- phDrop is the base quantity of pheromone dropped on each edge, it is
modified by gamma.
- Obviously, phDrop depends on the number of ants, the max size of
paths and the evaporation.
- minPh is 0.1 (so that each edge always has some pheromone).
- maxPh is 1 and should probably remains at this constant.
- Added a larger test with a grid.
- Now ants that reach a dead-end die and a new one appears on the nest.