Make all the Python files importable

For Sphinx documentation we need the Python files to be importable. The way to do this is to;

* Put all the current stuff under a `def main(args)` function.
* Put the following at the end;
```python
if __name__ == "__main__":
import sys
sys.exit(main(sys.argv))
```

The argument set up should remain at the top level so it can be automatically converted to help documentation.