Some small tweaks to facilitate versioning

I moved the version number from the code comments to `__init__.py`. Now there is just one location to indicate the version. Ideally a release tag should be made every time that the version is incremented. I already tagged `release/0.4.1`, and then immediately bumped to `release/0.4.2~pre1`. (I'm not entirely sure whether you can see my tags, though.)