Enable custom metrics in AutoEmulate compare loop

[sgreenbury]

Contributes towards #882.

This PR:

• Revises metrics to be stored by class in AutoEmulate enabling custom metrics to be used without maintaining a registry
• Updates base Metric API with required methods
• Updates first metric to be from list in order of population instead of unordered set
• Does not handle serialization and deserialization to disk of custom metrics:
  • e.g. path = ae.save(ae.best_result()); ae.load(path) will fail for custom metrics currently since the class is not deserializable from a string without either a pickling/registry approach
  • Relates to Further refactors to model & metadata save and load #631

[radka-j]

Looks great!