Default data

• Moved all variable references for validation on model_generator.py to a dedicated defaults.py file. This modification is needed to keep things separated, namely the default values that are considered for the model.
• Removed unused variables from the model_generator.py that were being sent to the front-end, namely the highest_const, emission_rate_per_person and exposed_occupants. These three variables were consuming time in the model generation and not being displayed. Therefore, with its removal we are saving some computational time.