Update scripts.py, clip(-10, 1) and penalize

• penalize unwanted NaNs #7 (closed)
• calc RPSS, then clip(-10,1) and penalize, then mean over forecast_time, spatially weighted mean [90N-60S], mean over lead_time and data_vars in skill_by_year and scorer #7 (closed)
• add assert functions in
  • scorer: https://renkulab.io/gitlab/tasko.olevski/s2s-ai-competition-scoring-image/-/merge_requests/2
  • skill_by_year
• Add notebook before commiting: git add current_notebook.ipynb
• rerun ML_train_and_predict.ipynb after new clip