add `prune_search_space`-method to optimization strategies

[SimonBlanke]

[SimonBlanke]

The idea of this feature is to reduce the size of the search-space after an optimization algorithm inside a strategy has finished its run. Before starting the next algorithm the pruning of the search-space can remove areas of the space that are distant from the best found position. This way the next algorithm can focus on the search-space round the best known position to find even better results.