Adding Retrofit

Added Retrofit for querying data from weather app

app/src/main/java/iastate/edu/weatherapp/ui/main/Retrofit/IOpenWeatherMap.java

+package iastate.edu.weatherapp.ui.main.Retrofit;
+
+import iastate.edu.weatherapp.ui.main.Model.WeatherResult;
+import io.reactivex.Observable;
+
+import retrofit2.http.GET;
+import retrofit2.http.Query;
+
+public interface IOpenWeatherMap {
+    @GET("weather")
+    Observable<WeatherResult> getWeatherByLatlng(@Query("lat") String lat,
+                                                 @Query("lon") String lng,
+                                                 @Query("appid") String appid,
+                                                 @Query("units") String unit);
+}
+