Ahmad Soori Laki

[FEATURE] Add multiprocessing support to backtest

3

Optimization of complex strategies takes too much time. Adding multiprocessing can solve the problem. `backtrader.cerebro` can be used. [definition](https://github.com/mementum/backtrader/blob/0fa63ef4a35dc53cc7320813f8b15480c8f85517/backtrader/cerebro.py#L80).