Add `zdd_ite`

[SSoelvsten]

Since we already provide ZDD operations with BDD semantics (i.e. zdd_top(), zdd_bot(), zdd_ithvar(x), and zdd_nithvar(x)), we may as well also provide the if-then-else operation. Currently, the end-user would have to fake it by computing (f & g) | (~f & h)

• [ ] Add zdd_ite by generalising bdd_ite into a common prod3 algorithm ( see also #148 )