def stop(task = :default)
      return nil unless tasks.keys.include?(task) && active?(task)
      time_taken = Time.now.to_f - tasks[task][:current].to_f
      tasks[task][:history] << { start: tasks[task][:current], stop: Time.now.to_f, time: time_taken }
      tasks[task][:current] = nil
      if retention && tasks[task][:history].size > retention then tasks[task][:history].shift end
      time_taken
    end
def stop(task = :default)
      return nil unless tasks.keys.include?(task) && active?(task)
      time_taken = Time.now.to_f - tasks[task][:current].to_f
      tasks[task][:history] << { start: tasks[task][:current], stop: Time.now.to_f, time: time_taken }
      tasks[task][:current] = nil
      if retention && tasks[task][:history].size > retention then tasks[task][:history].shift end
      time_taken
    end
Stop the referenced timer.

 @param [Symbol] task The name of the task to stop.
 @return [Float, NilClass] The amount of time the task had been running or nil if no matching task was found.