docs: public static DMatrixRMaj extractRow(DMatrixRMaj a , int row , DMatrixRMaj out ) {
          if( out == null)
              out = new DMatrixRMaj(1,a.numCols);
          else if( !MatrixFeatures_DDRM.isVector(out) || out.getNumElements() != a.numCols )
              throw new MatrixDimensionException("Output must be a vector of length "+a.numCols);
  
          System.arraycopy(a.data,a.getIndex(row,0),out.data,0,a.numCols);
  
          return out;
      }