add option to add books manually, fix multiple user selection bug

src/schemas/user.py

@@ -11,3 +11,13 @@ class User:
 
     def __repr__(self):
         return f"Name: {self.username}\nMatrikelnr.: {self.userid}\neMail: {self.email}"
+
+    def match(self, testuser: "User"):
+        name = testuser.username
+        user_id = testuser.userid
+        email = testuser.email
+
+        if name == self.username and email == self.email and user_id == self.userid:
+            return True
+        else:
+            return False