Add split train test method

Add test

Add split train test test method

	splitTrainTestFrom:usingTargetColumn:withProportions: aCollectionOfProportions to split aCollection into multiple sets. The sets to split are determined by the size of aCollectionOfProportions, which looks like #(0.7 0.3), and commonly it represents splitting between a training and test set.

Add AIPartitionedDataSet class to hold the splitted data from train and test set.