Made `Matrix<Scalar>` conform to `Collection` (row-wise, just like it already does with `Sequence`)

Sources/Surge/Matrix.swift

@@ -206,6 +206,30 @@ extension Matrix: Sequence {
     }
 }
 
+// MARK: - Collection
+
+extension Matrix: Collection {
+    public subscript(_ row: Int) -> ArraySlice<Scalar> {
+        get {
+            let startIndex = row * columns
+            let endIndex = startIndex + columns
+            return self.grid[startIndex..<endIndex]
+        }
+    }
+    
+    public var startIndex: Int {
+        return 0
+    }
+    
+    public var endIndex: Int {
+        return self.rows
+    }
+    
+    public func index(after i: Int) -> Int {
+        return i + 1
+    }
+}
+
 extension Matrix: Equatable {}
 public func ==<T> (lhs: Matrix<T>, rhs: Matrix<T>) -> Bool {
     return lhs.rows == rhs.rows && lhs.columns == rhs.columns && lhs.grid == rhs.grid