cleanup AxisCollection methods

[gdementen]

Currently, it is a huge mess. There is a mixture of inplace methods and methods returning a new AxisCollection.

I think we should make sure all methods are returning a new AxisCollection by default, and optionally implement an inplace argument for the methods where we really need an inplace version.

see also #783

[gdementen]

the worst offenders are the add & remove axes methods & operators: union/__add__/__or__/extend/append/insert
and __sub__/without/pop

[gdementen]

Part of the problem comes from an unclear data structure. We offer an ordereddict-like interface with some set-like flavour on top, but must also support duplicate names so we are no truly dict or set-like.

[gdementen]

References:
https://github.com/mlenzen/collections-extended
https://www.oreilly.com/learning/introducing-pandas-objects