Removed Descriptor

Not necessary anymore, replaced with SingleOutputOp<?>
Interfaces now extend SingleOutputOp<?>

Since Feature/Descriptor were removed, all interfaces in the statistics
package now implement SingleOutputOp
IRT implementations now extend AbstractFunction

Since Feature was removed, IRT implementations now implement
AbstractFunction
Removed "extends Feature"

Since Feature was removed, the generic implementations now longer
extend AbstractFeature
Removed parameterization

Since Percentile is a SingleOutputOp<DoubleType>,
Percentile<DoubleType> is now longer needed
Median now extends AbstractFunction