refactoring pharmacy

[AmirDahmouni]

Changelog :

-implement updateDrug method inside the drug class replace the nested if conditions with simple swicth/case to reduce complexity ( we could add new drug without reread all the previous code )
-implement increment and decrement benefit with conditions to assure that every benefit in interval 0-50
-add unit test for each drug

Instructions

-run npm install before start for testing
-run npm test
-run node index.js