Cdx based oldest newest and near (#159)

* implement oldest newest and near methods in the cdx interface class, now cli uses the cdx methods instead of availablity api methods.

* handle the closest parameter derivative methods more efficiently and also handle exceptions gracefully.

* update test code

tests/test_cdx_snapshot.py

@@ -41,3 +41,4 @@ def test_CDXSnapshot() -> None:
     )
     assert archive_url == snapshot.archive_url
     assert sample_input == str(snapshot)
+    assert sample_input == repr(snapshot)