Make some axistags functions private (was: insert axistags should increase the dimension of the array) about vigra

[ukoethe]

There is currently no two-way communication between arrays and their axistags. The rule is: always modify the array, then the axistags are automatically modified accordingly, e.g.:

b = array[..., numpy.newaxis]

creates a singleton axis and sets the corresponding axistag to '?' ( for 'unknown'). We may want to extend newaxis to allow for the specification of an axistag object:

b = array[..., vigra.newaxis(vigra.AxisInfo(...))]

array.squeeze() also works as expected. We should mark the dangerous axistags functions as private (leading underscore or so).

We may also consider if two-way communication is actually required.

[ukoethe]

I didn't make axistags functions private, but added remarks to this effect in the documentation.