allow to use TopologyGenerators with any kind of node model

[darrivau]

In TopologyGenerator only "default" mode of Node is used to generate the topologies. Is it possible to add a String parameter to every static generateXXX functions that could be "default" by default but could be used to specify another Node model?

[remikey]

Merged into develop.

What's been done:

• created Topology.DEFAULT_NODE_MODEL_NAME constant for external classes use
• renamed topology.TopologyGenerator into topology.TopologyGenerators
• removed topology.TopologyGeneratorFactory and exposed its inner generators in topology.generators.*

Note:

• the current package naming might be modified later

[remikey]

Merged into master.