Axis[slice_LGroup_on_incompatible_axis] has wrong behavior

In the same vein as #1117 and #1146, we have:

```python
>>> a = Axis('a=a0,a1,a2')
>>> alt_a = Axis('a=a0,a1')
>>> print(alt_a[:])     # <-- OK
['a0' 'a1']
>>> print(a[alt_a[:]])  # <-- BAD !
['a0' 'a1' 'a2']
>>> # we should .eval() the group
>>> print(a[alt_a[:].eval()])
['a0' 'a1']
```
