support Array.insert when label is not given

[gdementen]

>>> arr = ndtest(3)
>>> arr.insert(-1, before='a1', label='before_a1')
a  a0  before_a1  a1  a2
    0         -1   1   2
>>> arr.insert(-1, before='a1')
KeyError: "axis 'a' not found in {}"

Here is what I would have expected.

>>> arr.insert(-1, before='a1')
a  a0  None  a1  a2
    0    -1   1   2

IIRC, I did this by design: if there is no label, it assumes the axis is present in "value". However, I regret this now as it is both annoying (*) and the error message is hard to understand (especially when this happens in some larger code). I took several minutes to figure out I had forgotten the label argument, so I expect other users could really have trouble figuring what the problem is.

(*) there are cases where you really don't care about the label: you just want the value inserted at the right place.