Specifying axis label issue #27

Here and below - it seems you never need the quantity object, you either need the value or the unit. So what I suggest is that here you actually define:

```
tick_world_coordinates_unit = tick_world_coordinates.unit
tick_world_coordinates_values = tick_world_coordinates.value
```

*before* this ``if`` block, i.e. around line 401-402. Then in this block you can do the hstack on the values. Then below you can loop over the ``tick_world_coordinates_values`` object and avoid having to do ``.value`` everywhere. Does this make sense?