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/Adding System Introspection.rst

@@ -119,7 +119,7 @@ our source code can tell what resources are available. If both ``log`` and
   :name: MathFunctions/CMakeLists.txt-target_compile_definitions
   :language: cmake
   :start-after: # add compile definitions
-  :end-before: # install libs
+  :end-before: # state
 
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@@ -136,7 +136,8 @@ Since we may be using ``log`` and ``exp``, we need to modify
   :caption: TODO 4: MathFunctions/mysqrt.cxx
   :name: MathFunctions/mysqrt.cxx-include-cmath
   :language: c++
-  :end-before: #include <iostream>
+  :start-after: #include "mysqrt.h"
+  :end-before: include <iostream>
 
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