add test case

tests/test_single_obs_key.py

@@ -0,0 +1,15 @@
+from ase.build import molecule
+import znh5md
+
+def test_single_obs_key_info(tmp_path):
+    # Test a special case where only the first config has the key
+    # which caused an error in the past
+    io = znh5md.IO(tmp_path / "test.h5")
+    water = molecule("H2O")
+    water.info["density"] = 0.997
+    io.append(water)
+    del water.info["density"]
+    io.extend([water for _ in range(5)])
+    assert len(io) == 6
+    assert len(io[:]) == 6
+    assert len(list(io)) == 6