Get the number of workers in each mode.

        This returns a dictionary where the keys are mode constants
        and the values are a simple integer count of the number of
        workers in that mode.

def mode_counts(self):
        '''Get the number of workers in each mode.

        This returns a dictionary where the keys are mode constants
        and the values are a simple integer count of the number of
        workers in that mode.

        '''
        modes = {self.RUN: 0, self.IDLE: 0, self.TERMINATE: 0}
        for worker_id, mode in self.workers().items():
            modes[mode] += 1
        return modes