Checkpoint: Added docs, improved ExchangeEntity

buisnesslogic/src/main/java/com/plannaplan/services/ExchangeService.java

@@ -24,6 +24,12 @@ public class ExchangeService {
         return this.repo.save(exchange);
     }
 
+    
+    /** 
+     * @param assignment Assignment to trade for
+     * @param group Desired group
+     * @return Optional with Exchange if exist
+     */
     public Optional<Exchange> checkForExchange(Assignment assignment, Groups group){
         return this.repo.checkForExchange(assignment, group);
     }