⚡ Improve compute table clearing after garbage collection

[burgholzer]

Whenever garbage collection in the DD package collects something from the real number unique table, quite a lot of the compute tables are invalidated/cleared at the moment.
This is (most likely) not necessary for some of these tables that take only a Node* as an input and return a CachedEdge, i.e., none of the operands contain a complex number from the real number unique table. Consequently, it should be fine to leave them as they are if only complex numbers have been collected.

Originally posted by @burgholzer in #628 (comment)