Cleanup axes initialization.

[perrette]

Right now a large number of possibilities exist to initialize axes, in term of nature (list, numpy arrays, Axis instances) and methods to pass them as argument (as a sequence of axes, or in the 1-D case just the one axis). This is all fine and handy, but is more error prone, as an axis as a list of elements, and a list of axes look pretty similar at first sight, programmatically (need to look at content to distinguish them). Introduce a few more checks and error messages, so that at least obvious errors can be treated, e.g. issue #35.

And possibly, sacrifice a few options for the sake of clarity.