Adding content to NumericFeatureFrame needs re-normalisation #109
Description
When you add new content to a NumericFeatureFrame on top of existing content, then the min max normalisation needs to be recalculated for all content. Currently the existing content and new content will have their own scale.
The TestNumericFeaturesFrame.test_adding_content_mixed test demonstrates the wrong behaviour.
rank_by_params should also be called score_by_params