Add `bdd_topvar(...)` and `zdd_topvar(...)`

[SSoelvsten]

When variable ordering is introduced the returned value of bdd_minvar(f) and zdd_minvar(A) becomes ambiguous - is it the lowest value in the level order, i.e. always the root level, or is it the minimal occuring level, i.e. the minimum over all variable levels. Currently, there is no difference, but when the variable order changes, we very much need to differentiate this.

• Add bdd_topvar(f) and zdd_topvar(A) and make bdd_minvar(f) and zdd_minvar(A) aliases.
• Add the missing unit tests for all three operations.