Specifying axis label issue #27

If I do something like
```python
tick_world_coordinates_unit = tick_world_coordinates.unit
tick_world_coordinates_values = tick_world_coordinates.value
if self.coord_type == 'longitude':
    tick_world_coordinates_values = np.hstack([tick_world_coordinates_values, 
                                               tick_world_coordinates_values + 360])
    tick_world_coordinates = tick_world_coordinates_values * tick_world_coordinates_unit
```
is that okay? If I put ```tick_world_coordinates_values = tick_world_coordinates.value``` before the ```if``` statement, I can't convert it to degrees like I was doing previously