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.
In commit 80f5d28813 (Tutorial: Update step 2 style, 2022-07-25,
v3.25.0-rc1~226^2) we replaced some uses of `USE_MYMATH` with `MY_MATH`.
Restore the former name for consistency with the rest of the tutorial.
