Add Multiply method for Double-Integer

* This is the important variation that is used when multiplying the
  input matrix with the seed matrix

src/SparseTransform/DataStructures/DoubleMatrix.cs

@@ -90,5 +90,28 @@ namespace DataStructures
             }
             return result;
         }
+
+        /// <summary>
+        /// Multiply this matrix with another matrix
+        /// </summary>
+        /// <param name="other">second matrix</param>
+        /// <returns>product matrix</returns>
+        public DoubleMatrix Multiply(IntegerMatrix other)
+        {
+            DoubleMatrix result = new DoubleMatrix(this.RowCount, other.ColumnCount);
+            for (int i = 0; i < this.RowCount; i++)
+            {
+                for (int j = 0; j < other.ColumnCount; j++)
+                {
+                    double s = 0;
+                    for (int k = 0; k < this.ColumnCount; k++)
+                    {
+                        s += this[i, k] * other[k, j];
+                    }
+                    result[i, j] = s;
+                }
+            }
+            return result;
+        }
     }
 }
\ No newline at end of file