Tutorial: Move step 10 to step 4

Shifts steps 4-9 to 5-10.

Help/guide/tutorial/Step10/tutorial.cxx

@@ -1,11 +1,15 @@
 // A simple program that computes the square root of a number
+#include <cmath>
 #include <iostream>
-#include <sstream>
 #include <string>
 
-#include "MathFunctions.h"
 #include "TutorialConfig.h"
 
+// should we include the MathFunctions header?
+#ifdef USE_MYMATH
+#  include "MathFunctions.h"
+#endif
+
 int main(int argc, char* argv[])
 {
   if (argc < 2) {
@@ -19,7 +23,12 @@ int main(int argc, char* argv[])
   // convert input to double
   const double inputValue = std::stod(argv[1]);
 
-  const double outputValue = mathfunctions::sqrt(inputValue);
+  // which square root function should we use?
+#ifdef USE_MYMATH
+  const double outputValue = mysqrt(inputValue);
+#else
+  const double outputValue = sqrt(inputValue);
+#endif
 
   std::cout << "The square root of " << inputValue << " is " << outputValue
             << std::endl;