Tutorial: Refactor MathFunctions code

Propagate the refactor in Step 10 MathFunctions through all of the
steps. Use MathFunctions/MathFunctions.cxx instead of Tutorial.cxx
to determine which sqrt library is called. Adds .h files which
correspond to their .cxx files by name.

Help/guide/tutorial/Complete/MathFunctions/MathFunctions.cxx

@@ -10,6 +10,7 @@
 namespace mathfunctions {
 double sqrt(double x)
 {
+// which square root function should we use?
 #ifdef USE_MYMATH
   return detail::mysqrt(x);
 #else