Allow adding units after a unit conversion

I'm not sure if this is a bug report or feature request, but I'd like the following to work if possible:

```
10 km in meters + 100 meters //this is an error, 'Parameter x contains a unit with undefined value'
(10 km in meters) + 100 meters //this works fine
```

'10 km in meters' produces a result in meters, so I'd expect to be able to add more meters to it without bracketing. But I guess there's a precedence issue.