Improve legibility of `bdd_support` (earlier `bdd_varprofile`)

[SSoelvsten]

It is not clear, that Adiar has support to obtain all variables in a BDD or ZDD.

Tasks:

• BDD:
  • [x] Move bdd_varprofile up next to bdd_minvar and bdd_maxvar in the BDD API.
  • [x] Rename bdd_varprofile into bdd_support.
  • [ ] Add a new overload bdd_support(f) that returns a bdd cube (see also: #533).
• ZDD:
  • [x] Move zdd_varprofile up next to zdd_minvar and zdd_maxvar in the ZDD API.
  • [x] Rename zdd_varprofile into zdd_support.
  • [ ] Add a new overload zdd_support(A) that returns a zdd point (blocked until #571 is resolved).

Additional context This was noticed by Anna Blume Jakobsen while browsing through the documentation.

[SSoelvsten]

Doing this, we can also add bdd_varprofile in the way it is used in BuDDy, i.e. where you get to read through the levels file.