Add unit to Metric axes

Axis don't have units for now, but they should.

One implementation is to store units as string in the metadata keyword of every axis. metadata is '' by default, and it would mean "no unit"

This could be dealt with in the Metric generic class.

Plots should also use this, not sure if inheriting from pre-existing plot() function of Hist is the solution, but the compare method also need that and doesn't necessarily use .plot()