Draft: Support multiple APIs

This MR implements Support new gwdatafind server APIs (#28) by refactoring gwdatafind.api into a subpackage, and implementing a decorator for the gwdatafind.ui methods that selects the xxx_path function from the right API module dynamically.

Also adds a new -A/--api option for the command-line interface.

gwdatafind/tests/test_main.py

@@ -256,6 +256,7 @@ def test_filename(mfindurl):
         urltype=args.url_type,
         on_missing='warn',
         host=args.server,
+        api=args.api,
     )
     out.seek(0)
     assert out.read().rstrip() == mfindurl.return_value[0]