generalize sumColsBy and sumRowsBy so they are not bound to floats

src/FSharp/Matrix.fs

@@ -161,12 +161,12 @@ module Matrix =
         f
 
     /// Returns the sum of the results generated by applying the function to each column of the matrix.
-    let inline sumColsBy (f: int -> Vector<float> -> float) (A: #Matrix<float>) =
-        mapCols f A |> Seq.fold (+) 0.0
+    let inline sumColsBy (f: int -> Vector<float> -> 'a) (A: #Matrix<float>) =
+        mapCols f A |> Seq.reduce (+)
     
     /// Returns the sum of the results generated by applying the function to each row of the matrix.
-    let inline sumRowsBy (f: int -> Vector<float> -> float) (A: #Matrix<float>) =
-        mapRows f A |> Seq.fold (+) 0.0
+    let inline sumRowsBy (f: int -> Vector<float> -> 'a) (A: #Matrix<float>) =
+        mapRows f A |> Seq.reduce (+)
 
     /// Iterates over all elements of a matrix.
     let inline iter (f: float -> unit) (A: #Matrix<float>) =