add benchmark on MatSym

Author: alxbilger

Sofa/framework/Type/src/sofa/type/MatSym.h

@@ -112,21 +112,14 @@ class MatSym : public VecNoInit<D * (D + 1) / 2, real>
     /// Write access to element (i,j).
     inline real& operator()(const int i, const int j)
     {
-        if (i >= j)
-        {
-            return this->elems[(i * (i + 1)) / 2 + j];
-        }
-        return this->elems[(j * (j + 1)) / 2 + i];
+        return const_cast<real&>(std::as_const(*this)(i, j));
     }
 
     /// Read-only access to element (i,j).
     inline const real& operator()(const int i, const int j) const
     {
-        if (i >= j)
-        {
-            return this->elems[(i * (i + 1)) / 2 + j];
-        }
-        return this->elems[(j * (j + 1)) / 2 + i];
+        const auto [a, b] = std::minmax(i, j);
+        return this->elems[(a * (a + 1)) / 2 + b];
     }
 
     /// convert matrix to sym