Namespace cleanup

I think this branch is ready to merge now.

BTW: I realized that we now also have a mechanism for doing string realization with an explicit namespace because you can use the somewhat internal `set_item` method:
```Python
G.var.set_item(slice(None), 'var + tau', namespace={'tau':20*ms})
```
I think that's good enough and might actually be useful in some fancy framework where you allow string-based initializations in a parameter-exploration setting, I guess.