Support Custom User Metrics

[EdwinB12]

• Allow the user to write their own metrics

[sgreenbury]

There are a couple of places that could be good to support additional metrics:

• autoemulate/autoemulate/core/compare.py

  Line 507 in a04fa07

  best_result = self.best_result()

• autoemulate/autoemulate/core/compare.py

  Line 391 in a04fa07

  np.mean(score).item() for score in scores

  - this one relates to Update metric choosing optimal hyperparameters #661 where mean plus one standard error was explored