add Models and Retrofit Client

...src/main/java/com/mobilecomputing/pecunia/model/Trip.java

Frontend/.idea/misc.xml

Frontend/app/build.gradle

...rc/main/java/com/example/mobileapp/model/Transaction.java

...d/app/src/main/java/com/example/mobileapp/model/Trip.java

...d/app/src/main/java/com/example/mobileapp/model/User.java

...java/com/example/mobileapp/networking/RetrofitClient.java

...in/java/com/example/mobileapp/networking/TripService.java

...in/java/com/example/mobileapp/networking/UserService.java