#269 Move freeing some BDDs + applying plant invariants to CIF/BDD plugin

• Best to review per commit. The code to apply plant invariants gets changed again in later commits (is done in multiple steps).
• Changes
  • Moved some code related to freeing BDDs from CifDataSynthesis to CifBddSpec. And made that robust against repeated invocation.
  • CifBddSpec now has a method to clean up all BDDs of the entire CIF/BDD specification. CifBddEdge has a method to clean all up all its BDDs as well.
  • Moved applying plant invariants from 'CifDataSynthesis' to a new CifBddApplyPlantInvariants class in the 'bdd' plugin.
  • Slightly changed/improved the debug output of applying plant invariants.

Addresses #269 Addresses #621