Use click group

Pull request

Follows:

• !61 (merged)

Please confirm that this pull request has done the following:

• Tests added

• Documentation added (where applicable)

  • Document how the environment variables can be used (N/A, see #51)

• Example added (either to an existing notebook or as a new notebook, where applicable)

• Description in CHANGELOG.rst added

Adding to CHANGELOG.rst

Please add a single line in the changelog notes similar to one of the following:

- (`!XX <https://gitlab.com/netcdf-scm/netcdf-scm/merge_requests/XX>`_) Added feature which does something
- (`!XX <https://gitlab.com/netcdf-scm/netcdf-scm/merge_requests/XX>`_) Fixed bug identified in (`#YY <https://gitlab.com/netcdf-scm/netcdf-scm/issues/YY>`_)