Maybe try to convert NodeProcess into an Interface for easier implementation

[n-peugnet]

I am not sure yet if this will be better with an interface than with an abstract class but we can try.

[n-peugnet]

After some thinking, we should

1. add two interfaces:
   • Infrastructure
   • NodeProcess
2. rename the current Infrastructure and NodeProcess into AbstractInfrastructure and AbstractNodeProcess
3. finally try to remove AbstractNodeProcess by removing the infra property and accessing the current Infrasctructure through a static method in Ppi (e.g. Ppi.infra())

The problem with (3.) is that, currently, I think (but not sure) PeersimInfrastructure is cloned so that each NodeProcess has its own instance. Anyway this could be changed as it could work with only one instance of PeersimInfrastructure