implement Axis.min and Axis.max

[gdementen]

or more generally make Axis as much like Array as possible. Maybe making Axis inherit from Array would be possible?

[alixdamman]

What to do when labels are not integers or numeric ?

[alixdamman]

Maybe making Axis inherit from Array would be possible?

So it is maybe the right time to implement some kind of super Abstract class for LArray objects (like pandas and xarray do)?
But that's another issue.