Possible bug on worker pool size

I believe there is an error at https://github.com/LGE-ARC-AdvancedAI/auptimizer/blob/master/src/aup/Proposer/hpbandster/core/dispatcher.py#L232 where `del ...` leaves a `None` in the `worker_pool` without ever reducing the size of the collection. I think `filter` or some other reduction might be needed when cleaning up.