Improved process callbacks

[arnobaer]

An optimized approach for custom process/thread callback registration and calls.

class Process(comet.Process):
    def run(self):
        while self.running:
            self.push("temp", 22.5)
            self.push("humid", 50.0)
            self.push("status", 42)

process = Process(
    fail=ctrl.handle_error, # built in callback
    temp=ctrl.handle_temp,
    humid=ctrl.handle_humid
)
process.callbacks["status"] = ctrl.handle_status
...

Optional move built in callbacks begin, finish and fail also to property callbacks to avoid accidental assignment in process run().