Rename `label` to `level`

[SSoelvsten]

Currently, Adiar only uses the identity variable ordering, i.e. where x_i has level i and so on. To support variable ordering without making the code incomprehensible, we need to replace label with level where appropriate.

• [ ] ptr / uid
• [ ] node
• [ ] all request structs?

[SSoelvsten]

Currently, I have given it to the deprecation label, but question is whether some parts of the public API will end up with a breaking change? It might be a good idea to already do this as part of #397 for v2.0 rather than v2.1 .

[SSoelvsten]

One may also consider, whether nodes should only know the label and not the level. Whenever a request is made, the label is translated into its level to still get the queue to work. The positive side-effects of this is, that this may allow one to do the swapping operation when combined with #154 .