updates on possibilities for dispersal on graphs. It is now ok to set D = nothing, which triggers a new dispersal kernel where individuals exactly move to neighbours

Modified increment for graph, since randomwalk return same vertex as the one provided when niter = 1

Many changes, we are almost send a full run. We need now to modify constructor of agent, to distinguish between AgentA and AgentNA.
Then we need to continue unit testing with simulation, maybe adding a callback function.
After that, we are good to go.