creating an Axis with mixed type does not work

[gdementen]

>>> Axis([0, "a"], "value")
Axis(['0', 'a'], 'value')

The only workaround I know is to force the dtype at the numpy level:

>>> Axis(np.array([0, "a"], dtype=object), "value")
Axis([0, 'a'], 'value')

[gdementen]

AFAICT, we will need to traverse all labels to fix this, which is most unfortunate for larger axes. One way to prevent traversing all labels would be to keep the labels as a list when given as a list, which would have the added bonus of being more efficient in some cases, but that would make the code more complex in a few places and could lead to subtle errors, so I think it is probably not worth it. I would have to think some more about the consequences of this (maybe keeping it as a list until we need it as an array would be doable -- be then we are back at the original issue).

[gdementen]

Implementing a dtype argument on Axis, would make the workaround less awful. But that would clearly not be enough (I don't want users to have to specify dtype manually, except maybe to gain performance)