Make a glossary of all terms used and simplify and homogenize the vocabulary

[gdementen]

Throughout the API and documentation, we use various terms for very similar or even the same things.

We should:

• make an exhaustive list of those terms
• make a glossary of them (even for us, having precise definitions would help to know when to use each term)
• determine which are most user-friendly (possibly poll our users about this)
• apply those changes everywhere so that their usage is consistent

Examples include:

• key vs subset vs selection vs Group vs label vs tick (on a axis) vs filter (as an action for the method name) vs boolean filter (to be used in getitem)
• axis vs dimension
• index vs position (including the name "reindex" which I never liked but I don't like relabel either 😉 )
• axis reference
• join method vs align vs ...
• ...

[gdementen]

I guess we should split this issue in several: creating a glossary of existing terms could be done relatively quickly, while reviewing the whole API is more an LArray 1.0 thing, which is probably several years away