converting tabs to spaces in cpu (#1439)

This PR converts tabs to white spaces.
The statement I used for the conversion:
```find . -name "*.[ch]" -exec zsh -c 'expand -t 4 "$0" > /tmp/e && mv /tmp/e "$0"' {} \;```
Afterwards, I had a quick overview of the converted files to prevent odd indentation.
Merge pull request #1514 from cgundogan/remove_tabs_cpu

converting tabs to spaces in cpu (#1439)