Fix (?) and generalise axistetra, legendtetra

[thomased]

Generalise axistetra, legendtetra so they work with the CIELAB plot and other (possible future) 3d plots.

[thomased]

Both are currently broken, possibly as a result of the margin-handling updates in 2.6.1 (TBC).

Can certainly put them on the to-fix list, but might also be worth thinking about the utility of the functions? I've never actually used them (for whatever that's worth...). Legends seem like the kind of thing that users could handle themselves if so desired. Axes are neat, but again I've never seen them used and the vertex markers do a lot of that work already. Hmm.

Just a thought anyway.