Ensure Model ID is unique

The ID of the newly created model was obtained as the length of the
array of models, but if some models "in the middle" were deleted, the
new ID could be the same as the existing one.

We are sure that the last ID is the maximum, so we just take +1 than the
last element.

howdy/src/cli/add.py

@@ -78,13 +78,16 @@
 # Set the default label
 label = "Initial model"
 
+# some id's can be skipped, but the last id is always the maximum
+next_id = encodings[-1]["id"] + 1 if encodings else 0
+
 # Get the label from the cli arguments if provided
 if builtins.howdy_args.arguments:
  label = builtins.howdy_args.arguments[0]
 
-# If models already exist, set that default label
-elif encodings:
- label = _("Model #") + str(len(encodings) + 1)
+# Or set the default label
+else:
+ label = _("Model #") + str(next_id)
 
 # Keep de default name if we can't ask questions
 if builtins.howdy_args.y:
@@ -106,7 +109,7 @@
 insert_model = {
  "time": int(time.time()),
  "label": label,
- "id": len(encodings),
+ "id": next_id,
  "data": []
 }