Convert tabs to spaces.

In my last attempt, I seem to have used a faulty find command that only picked
up some header files, not all the files I had intended to convert. Here's
another attempt.

```
find . -name '*.cpp' ! -type d -exec bash -c 'expand --tabs 4 "$0" > /tmp/e && mv /tmp/e "$0"' {} \;
find . -name '*.h' ! -type d -exec bash -c 'expand --tabs 4 "$0" > /tmp/e && mv /tmp/e "$0"' {} \;
find . -name '*.c' ! -type d -exec bash -c 'expand --tabs 4 "$0" > /tmp/e && mv /tmp/e "$0"' {} \;
find . -name 'CMakeLists.txt' ! -type d -exec bash -c 'expand --tabs 4 "$0" > /tmp/e && mv /tmp/e "$0"' {} \;
```

@sherm1 thanks for your comments on my recent PR, as they helped me discover that I was unsuccessful at converting tabs to spaces in all the files I intended.