Add IWeigtedLearner and IWeigtedIndexedLearner interfaces to Common.Interfaces

[mdabros]

Add interface for learners supporting sample weights:

IPredictorModel<TPrediction> Learn(F64Matrix observations, double[] targets, 
double[] sampleWeights);

Add interface for learners supporting sample indices and sample weights:

IPredictorModel<TPrediction> Learn(F64Matrix observations, double[] targets, 
double[] sampleWeights, int[] indices);