refactor AddNode* methods in swarm/network/simulation

[acud]

as discussed in #1057, our methods of adding nodes into a simulation are spaghetti and have to be cleaned up and refactored, with the appropriate functionalities moved to p2p/simulations package

[frncmx]

The original idea was to move node.go from swarm/network/simulation to p2p/simulations. The methods in node.go currently belong to type Simulation, those methods would belong to type Network after the refactor. However, Network has too many methods already and we would double their count (see picture). That we don't want.

As I noticed, most of the methods in node.go and connect.go are not in use. I considered those as dead code and tried to remove in #1076. However the removal was voted down, as we consider the methods as part of a test helper library.

Another idea from @justelad was to simplify the interface to something like:

Connect(ids []enode.ID,  topology Topology)
AddNodes(count int, topology Topology, opts ...AddNodeOption)

But we agreed with @zelig; we don't want to spend that effort on this right now.