Refactored Code in User Model

Updated User Model
- refactored all attribute scores to one "score_to_gpa" to simplify code  
- tested with original and same numbers received and following errors recieved....may need to update tests
rspec ./spec/models/user_spec.rb:347 # User#sleep_score_to_gpa converts gpa to an integer
rspec ./spec/models/user_spec.rb:338 # User#movement_score_to_gpa converts gpa to an integer
rspec ./spec/models/user_spec.rb:329 # User#nutrition_score_to_gpa converts gpa to an integer

I can revert to previous version if recommended.