General improvements for GM handling

[fgrsnau]

This PR handles two things:

• It allows to save (reparametrized) GMs in OpenGM container format (using ExplicitFunctions).
• Tweaks ProblemConstructor to match LP_MPs interface (only small inconsistency I recognized during writing temporary code).

Saving GMs is a bit complicated, as some models have TripletFactors and some have not. If you have better ideas than introducing a MrfHasTripletFactors trait, please let me know :)

[fgrsnau]

I replaced the TripletFactor trait by a constexpr method.