Fix FutureWarning about using `... == None`

Hello,

This PR fixes the warning:

```
/Users/martin/shortbite/engine/venv/lib/python2.7/site-packages/pyamg/util/linalg.py:194: FutureWarning: comparison to `None` will result in an elementwise object comparison in the future.
  if initial_guess == None:
```

Changed to use `if initial_guess is None:` instead.