Return correct metadata when loading observations

src/pypfilt/obs.py

@@ -450,16 +450,11 @@ class Univariate(Obs):
         # Load the data table.
         fields = [io.time_field(time_col), (value_col, np.float64)]
         df = io.read_fields(time_scale, filename, fields)
-        # Rename the columns to 'time' and 'value'.
-        rename_to = {
-            time_col: 'time',
-            value_col: 'value',
-        }
-        new_names = tuple(
-            rename_to.get(name, name) for name in df.dtype.names
-        )
-        df.dtype.names = new_names
-        return df
+        # Rename the columns to 'time' and 'value', and ensure that the dtype
+        # metadata is updated to reflect the renamed columns.
+        fields_out = [io.time_field('time'), ('value', np.float64)]
+        dtype_out = io.fields_dtype(time_scale, fields_out)
+        return df.astype(dtype_out)
 
     def row_into_obs(self, row):
         """