The namespace is important because a repr typically looks like this:

```
Unit('m')
```

You need a namespace in which to look up `Unit`.  Try `eval("Unit('m')"` in the console.  It won't work.

There are cases in which we will want `__str__` to roundtrip, but not `__repr__` -- for example if the data is very large, so I think it's correct to be able to turn one or the other off.