`Axis` should be a subtype of `IXYZ`

[thierry-martinez]

Currently, Axis and IXYZ are defined as two distinct Enum types, leading to a preference for IXYZ in contexts where Axis would be more appropriate but would require an undesirable cast. See this discussion for instance.

Ideally, we should redefine IXYZ as Literal[I] | Axis.

[EarlMilktea]

@thierry-martinez Additionally, I suggest renaming Axis -> XYZ for coherence.