Use warnings.warn instead of logging.warn when loading Problem from H…

…DF5 without an Objective (#1253)

Allows using warnings filter to get rid of this message.


```python
import warnings
warnings.filterwarnings("ignore", category=UserWarning, message=".*You are loading a problem.*", module="pypesto.store.read_from_hdf5")
```

pypesto/store/read_from_hdf5.py

@@ -2,6 +2,7 @@
 
 import ast
 import logging
+import warnings
 from pathlib import Path
 from typing import Union
 
@@ -120,10 +121,9 @@ def read(self, objective: ObjectiveBase = None) -> Problem:
         if objective is None:
             objective = Objective()
             # raise warning that objective is not loaded.
-            logger.info(
-                'WARNING: You are loading a problem.\nThis problem'
-                ' is not to be used without a separately created'
-                ' objective.'
+            warnings.warn(
+                'You are loading a problem. This problem is not to be used '
+                'without a separately created objective.'
             )
         problem = Problem(objective, [], [])