Fix member ref base type

- was incorrectly calling genfunc.execute() instead of genfunc
- Fixes #313

test/NthDerivative/CustomDerivatives.C

@@ -148,20 +148,20 @@ float test_exp_darg0(float x);
 float test_exp_d2arg0(float x);
 
 int main() {
-    auto fn1 = clad::differentiate<2>(test_sin);
-    printf("Result is = %f\n", fn1.execute(0.5)); // CHECK-EXEC: Result is = -0.479426
+    clad::differentiate<2>(test_sin);
+    printf("Result is = %f\n", test_sin_d2arg0(0.5)); // CHECK-EXEC: Result is = -0.479426
 
-    auto fn2 = clad::differentiate<2>(test_cos);
-    printf("Result is = %f\n", fn2.execute(1.0)); // CHECK-EXEC: Result is = -0.540302
+    clad::differentiate<2>(test_cos);
+    printf("Result is = %f\n", test_cos_d2arg0(1.0)); // CHECK-EXEC: Result is = -0.540302
 
-    auto fn3 = clad::differentiate<2>(test_trig, "x");
-    printf("Result is = %f\n", fn3.execute(0.5, 1.0, 1, 2)); // CHECK-EXEC: Result is = -2.364220
+    clad::differentiate<2>(test_trig, "x");
+    printf("Result is = %f\n", test_trig_d2arg0(0.5, 1.0, 1, 2)); // CHECK-EXEC: Result is = -2.364220
 
-    auto fn4 = clad::differentiate<2>(test_trig, "y");
-    printf("Result is = %f\n", fn4.execute(1.0, 0.5, 2, 1)); // CHECK-EXEC: Result is = -0.060237
+    clad::differentiate<2>(test_trig, "y");
+    printf("Result is = %f\n", test_trig_d2arg1(1.0, 0.5, 2, 1)); // CHECK-EXEC: Result is = -0.060237
 
-    auto fn5 = clad::differentiate<2>(test_exp);
-    printf("Result is = %f\n", fn5.execute(2)); // CHECK-EXEC: Result is = 982.766663
+    clad::differentiate<2>(test_exp);
+    printf("Result is = %f\n", test_exp_d2arg0(2)); // CHECK-EXEC: Result is = 982.766663
 
 }