Save/Load operations not fully supported

[khosravipasha]

Keeping track of what's needed

• Cannot save circuit without vtree (workaround: use infer_vtree to do that after fixing its limitation non-smooth circuits: Bug in isstruct_decomposable, infer_vtree LogicCircuits.jl#47)
• Cannot save circuit that is not struct decomposable, need to choose a format for those cases.
• Cannot save CNF/DNFs (can only load them)
• Save a Prob/Logistic/Regression circuit without vtree
• Saving/Loading of regression circuits (now mostly support logistic circuits only)
• Save/Load mixture models, (eg from strudel). See Studel missing APIs. Save/load/MAR/etc #28 for suggestions.

For the load/save of Strudel (mixtures), one suggestion is that save all related files and metadata in a single Zip file (with appendix .strudel). We need the structure, the vtree, the mixture weights, and paramters. (maybe more). One option is using ZipFile.jl. Also see Juice-jl/LogicCircuits.jl#12

[khosravipasha]

Also related to Tractables/LogicCircuits.jl#40

[guyvdbroeck]

The first 3 check boxes above are resolved in Tractables/LogicCircuits.jl#99
The remaining ones should be moved to https://github.com/Juice-jl/ProbabilisticCircuits.jl

[khosravipasha]

@guyvdbroeck Last one was saving strudel models whichI think we have now in `ensemble_io.jl"?
so closing this.