fixed kNN for kNNBasic on issue #131

[ODemidenko]

currently I introduced all the changes only to one kNN algo. If you ok with this implementation - I immediately proceed with the others

• Move "Compute similarity" from AlgoBase into SymmetricAlg (as it belongs to kNN only)
• implement common logic for fixed kNN
• inject new logic into kNNBasic, as optional behaviour
• test new behaviour vs teacher examples from Coursera course
• inject new logic into kNNWithMeans, as optional behaviour
• inject new logic into kNNBaseline, as optional behaviour