STY: tabs -> space to silence build warnings.

Gets rid of the nonconforming tab warnings when building with `-Wall`. I don't know if it's desirable to have style changes in the source like this. I just ran

```
find . -name *.f ! -type d -exec bash -c 'expand -t 4 "$1" > /tmp/expand && mv /tmp/expand "$1"' _ {} \;
```