Help: Populate tutorial guide text

Migrate tutorial text from individual `directions.txt` files to the main
tutorial document.  Add some comments to source code to provide anchors
for inclusion.

Help/guide/tutorial/Step3/CMakeLists.txt

@@ -6,7 +6,7 @@ set(CMAKE_CXX_STANDARD 14)
 # should we use our own math functions
 option(USE_MYMATH "Use tutorial provided math implementation" ON)
 
-# the version number.
+# set the version number
 set(Tutorial_VERSION_MAJOR 1)
 set(Tutorial_VERSION_MINOR 0)
 

Help/guide/tutorial/Step3/directions.txt

@@ -1,26 +0,0 @@
-# Adding Usage Requirements for Library #
-
-Usage requirements allow for far better control over a library / executable's
-link and include line. While also giving more control over the transitive
-property of targets inside CMake. The primary commands that leverage usage
-requirements are:
-
-  - target_compile_definitions
-  - target_compile_options
-  - target_include_directories
-  - target_link_libraries
-
-First up is MathFunctions. We first state that anybody linking to MathFunctions
-needs to include the current source directory, while MathFunctions itself
-doesn't. So this can become an INTERFACE usage requirement.
-
-Remember INTERFACE means things that consumers require but the producer doesn't.
-
-  target_include_directories(MathFunctions
-            INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
-
-Now that we've specified usage requirements for MathFunctions we can safely remove
-our uses of the EXTRA_INCLUDES variable.
-
-Run cmake or cmake-gui to configure the project and then build it with your
-chosen build tool.

Help/guide/tutorial/Step3/tutorial.cxx

@@ -5,6 +5,7 @@
 
 #include "TutorialConfig.h"
 
+// should we include the MathFunctions header?
 #ifdef USE_MYMATH
 #  include "MathFunctions.h"
 #endif
@@ -12,6 +13,7 @@
 int main(int argc, char* argv[])
 {
   if (argc < 2) {
+    // report version
     std::cout << argv[0] << " Version " << Tutorial_VERSION_MAJOR << "."
               << Tutorial_VERSION_MINOR << std::endl;
     std::cout << "Usage: " << argv[0] << " number" << std::endl;
@@ -20,6 +22,7 @@ int main(int argc, char* argv[])
 
   double inputValue = std::stod(argv[1]);
 
+  // which square root function should we use?
 #ifdef USE_MYMATH
   double outputValue = mysqrt(inputValue);
 #else