indexing using X.axis[key] or 'axis[key]' syntaxes must not try other axes

[gdementen]

>>> arr = ndtest(3)
>>> arr
a  a0  a1  a2
    0   1   2
>>> # this should NOT succeed
... arr[X.b['a1']]
1
>>> # neither should this
... arr['b[a1]']
1
>>> arr = ndtest((2, 3))
>>> # this fails as it should
>>> arr[X.b['a1']]
ValueError: X.b['a1'] is not a valid label for any axis

The above issue should not be too hard to fix because I don't see a use case we would break by restricting that.