Created customizable ComputingNodesGenerator + small fixes

The ComputingNodesGenerator class is now customizable similarly to TopologyCreator, SimulationManager, etc., classes. I believe this is a useful modification that allows the simulator users to have more control in how the resources are generated, improving the extensibility of the simulator. For example, in my own use case, I needed to provide more information about the edge datacenters than is specified in the XML setting file, and the simplest way to do this was to add my own fields into the XML file and then handle the added fields in the generator. Further, in some use cases users may want to have more control over how the edge devices are initially placed on the simulation area
A couple small fixes:
- In NetworkLink.java file, line 177, zero is added to the task's network time. I changed this so that it adds the link's latency, which is evidently what should be added.
- When placing the edge devices in the computing node generator, both x and y coordinates are randomly drawn based on the simulation map length. I changed this so that the x coordinate is randomly drawn based on the simulation map width.

CharafeddineMechalikh / PureEdgeSim