Make it possible to define axes labels of an ArrayDef variable after class instantiation

[alixdamman]

Also, I think it that it would be useful to provide a way for users to declare that an array must have some specific axes even if the labels of those axes are not specified yet. In the example above, this would mean the ability to write something like either:

population_obs = ArrayDef((AGE, GENDER, "year"))

or better yet (if possible -- I think it would be tricky but possible):

population_obs = ArrayDef((AGE, GENDER, obs_years))

the later might require using AxisDef instances instead of just "Axis" to be able to differentiate instances: eg:

obs_years = AxisDef("time")
population_obs = ArrayDef((AGE, GENDER, obs_years))

The goal is keep the class definition as close as possible to a complete reference of what each variable is (and also be more precise in the safety checks)

Originally posted by @gdementen in #840