Add import/export

[SHoltzen]

Updating this to be a tracking issue for the standardized import/export of all the main datastructures within the library.

All these import/export tools can be found in src/serialize/

• Serialize/Deserialize BDD
• Serialize/Deserialize SDD
• Serialize/Deserialize d-DNNF
• Serialize/Deserialize VTRee
• Serialize/Deserialize DTree
• Serialize/Deserialize Interaction Graph
• Serialize/Deserialize weighted model counting parameters

In addition to serializing/deserializing these data structures, it's necessary to be able to load them into the relevant builder/manager. Here we track these workflows:

• Import/Export BDD from builder
• Import/Export SDD from builder
• import/Export d-DNNF from builder