Add Losses (and Penalties) modules or group under Objectives module

[pat-alt]

Instead of relying on Flux.Losses, in the end it seems more appropriate to have a separate module for custom loss functions. Similarly, it may be useful to have a designated module for penalties. Not sure yet, but perhaps the best option is to group both under an Objective module.

• Decide on architecture
• Implement custom losses
• Move existing penalty methods
• Add an @objective macro for composable generators
• Be explicit about convergence type: threshold, max_iter or gradient.
• Add additional macros for composable generators.
• Update docs.
• Augment tests.

[pat-alt]

Some thoughts after completing #121

• It may be worth linking penalties and evaluation measures, since the two are in many case related: L1/L2-norm for example is used both as a penalty and evaluation measure.
• Should every penalty be usable as an evaluation measure? Both take a ce::CounterfactualExplanation as their main argument ... Edit: No, doesn't work for CCE, for instance.