tidy create-sw-index.py

Things to do:
1) if dir of sw files is not given, then default to '.'
2) move from tools to semantic_db, and rename, then we can invoke using:
```
python3 -m semantic_db.create_index path-to-sw-files
```
3) tighten the import up so we only import extract_sw_stats() from code.py: Currently we use:
```
from semantic_db import *
```
4) test it works