deprecate Axis(pd.Index())

[gdementen]

in _to_ticks we have:

if isinstance(s, pd.Index):

which is not used internally but is useful if one wants to create an Axis from a pandas Index manually.

I don't think this should be allowed directly in la.Axis (we accept too many things in there) and would rather have a dedicated class method for that. Unsure even that is worth it.

In any case, this code path has no test coverage, so if we decide to continue supporting it as-is, it needs a test.

[gdementen]

I think we should just deprecate it and if someone complains, move the functionality to Axis.from_index()