Improving binary feature processing
Created by: Dreamlone
Modified the Imputer so that it can correctly handle integer features with unique values of 2 or less.
For example, now [1, 1, nan, 0] will not be transformed into [1, 1, 0.75, 0], but into [1, 1, 1, 0].
Another case: [1, nan, nan, 0] (the number of known categories is equal to each other). Then it will be converted inro [1, 0.5, 0.5, 0]
Minor issues with preprocessing and processing of multi-output cases have also been corrected