Fix pandas deprecation warning (#896)

Fixes
```
test/base/test_history.py: 229 warnings
  /home/runner/work/pyPESTO/pyPESTO/pypesto/objective/history.py:775: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
    self._trace = self._trace.append(row)
```

pypesto/objective/history.py

@@ -773,7 +773,9 @@ def _update_trace(
             else:
                 row[(var, float('nan'))] = np.NaN
 
-        self._trace = self._trace.append(row)
+        self._trace = pd.concat(
+            (self._trace, pd.DataFrame([row])),
+        )
 
         # save trace to file
         self._save_trace()