Improve ensemble selection memory usage

[mfeurer]

• separate storage of data inside the ensemble builder into two dictionaries to separately store them on disk (one for scores and one for predictions). If we run over the allocated memory, we can still make use of the stored scores
• do not stop the ensemble builder when the number of models to consider can no longer be reduced so that the ensemble builder can still delete models from the hard drive if necessary.
• avoid unnecessary memory copies during ensemble construction
• improve structure of temporary directories during unit testing