Enable exporting objective function settings to a file

[AndreFCruz]

Summary

• Currently if we load a model from a file it will look for the "constrained_cross_entropy" objective function and will not find it.
• We need to export the objective function settings to a file (serialize the objective settings) in order for the model to load up all necessary definitions.
• This makes it so currently we can't resume training of a model loaded from a file (but we can get predictions from that model).

Related to #36