Move executor field from Algorithm to PopulationEvaluator

[itaitzruia4]

Currently, the algorithm receives executor and max_workers fields in its constructor, initializes the Executor object, and passes it to PopulationEvaluator, but only the PopulationEvaluator activates it.
The only reason that it is in Algorithm, is to be able to call executor.shutdown at the end of the run.

I suggest moving this field entirely to PopulationEvaluator and adding a termination method to the evaluator, which will be called from the algorithm at the end of the run.

This issue will probably be addressed along with #63.