A few new features

• Can schedule processes with priorities.
• Multiple manager runners

  // Run for a certain amount of time
  Manager& run(high_resolution_clock::duration runtime);
  
  // Run indefinitely or until halt() (TODO: needs C interrupt handler)
  Manager& run();
  
  // Run until a condition becomes true
  Manager& run(std::function<bool()> condition);

  The last method was required to get some tests to pass. For example, the priority methods ran for 100 ms, but on Dockerhub, not much happens in 100ms.
• A new halt() method in processes, which tells the manager to stop, calling all processes' stop methods.
• A flag that should recognize Raspberry pi v3 B in the Makefile.
• A method in State for getting a pointer to the containing state machine.