PADAS/er-client: Method additions for add/delete

What's Changed fix:removing token_url as required kwarg, hardening auth behavior, adding required client_id kwarg to docs and examples. by @JoshuaVulcan in https://github.com/PADAS/er-client/pull/47 Add add_subjects_to_subjectgroup to AsyncERClient by @victorlujanearthranger in https://github.com/PADAS/er-client/pull/49 version to 1.14.0 by @chrisdoehring in https://github.com/PADAS/er-client/pull/50 feat: add async_mode param to AsyncERClient.delete_source by @victorlujanearthranger in https://github.com/PADAS/er-client/pull/51 New Contributors @victorlujanearthranger made their first contribution in https://github.com/PADAS/er-client/pull/49 Full Changelog: https://github.com/PADAS/er-client/compare/v.1.12.0...v1.15.0