Changed deprecated DataFrame.append to pd.concat to fix warning (#1487)

lso removed a loop that wasn't doing anything. DataFrame.append doesn't operate in place so that second loop wasn't causing any side effects, and it was also just adding empty series anyway. If anyone has any insight about why that was there in the first place that would be nice.

autosklearn/metalearning/metalearning/meta_base.py

@@ -73,11 +73,7 @@ def add_dataset(self, name, metafeatures):
                 "Dataset %s already in meta-data. Removing occurence.", name.lower()
             )
             self.metafeatures.drop(name.lower(), inplace=True)
-        self.metafeatures = self.metafeatures.append(metafeatures)
-
-        runs = pd.Series([], name=name, dtype=float)
-        for metric in self.algorithm_runs.keys():
-            self.algorithm_runs[metric].append(runs)
+        self.metafeatures = pd.concat([self.metafeatures, pd.DataFrame(metafeatures).T])
 
     def get_runs(self, dataset_name, performance_measure=None):
         """Return a list of all runs for a dataset."""