Update data_reader_obs.py removing asserts (ecmwf#817)

Author: sophie-xhonneux

src/weathergen/datasets/data_reader_obs.py

@@ -53,9 +53,9 @@ def __init__(self, tw_handler: TimeWindowHandler, filename: Path, stream_info: d
         t_chs_exclude = stream_info.get("target_exclude", [])
 
         source_n_empty = len(s_chs) > 0 if s_chs is not None else True
-        assert source_n_empty, "source is empty; at least one channels must be present."
+        # assert source_n_empty, "source is empty; at least one channels must be present."
         target_n_empty = len(t_chs) > 0 if t_chs is not None else True
-        assert target_n_empty, "target is empty; at least one channels must be present."
+        # assert target_n_empty, "target is empty; at least one channels must be present."
 
         self.source_channels = self.select_channels(data_colnames, s_chs, s_chs_exclude)
         self.source_idx = np.array([self.colnames.index(c) for c in self.source_channels])