Replace simulation manager interface with a builder

[anderspkd]

The sim::Manager interface is a bit bothersome to handle from a user's point of view.
A much better design would be to construct a Manager using a builder. This has a number of benefits:

• No need to pass around an std::unique_ptr for the manager.
• Easier to customize the simulation. E.g., no need to subclass some particular subclass of sim::Manager and then pass an std::ostream object to the constructor in order to output results to a file. Something like builder.setOutput(some_stream) should do the trick.
• Allows making the object that the builder builds (the manager) be responsible for also running the simulation.

Builder proposal:

class SimulationBuilder {
public:
  static SimulationBuilder create();
  SimulationBuilder& outputTo(const std::string& filename);
  SimulationBuilder& outputToStdout();
  template <typename FUNC>
  SimulationBuilder& protocolCreator(FUNC&& creator);
  SimulationBuilder& networkConfig(...);
  
  Simulator build();
};

auto builder = SimulationBuilder::create();
// bla bla

auto simulator = builder.build();

// runs the simulation
simulator.run();