Datalad-xnat

- Minimal port of the test battery to pytest.

- Bump dependency on DataLad to v0.18, for pytest compatible test helpers.

- Minor updates of CI setup and packaging, following updates in the
DataLad extension template.

- The first actual release of this extension. It is the outcome of Juelich
Brain Hackathon 2021, with numerous contributors:
https://github.com/datalad/datalad-xnat#contributors-

In particular it features:

- Authenticated and anonymous access to XNAT

- Support for major deployments: ConnectomeDB, NITRC IR, XNAT Central

- Full or constrained exports of entire instances, individual projects,
single subjects, particular acquisitions, and limit by resource
collections

- Flexible dataset structuring with support for multiple layers of
nesting

- Parallel downloads

- Documentation

- Roughly 90% test-coverage

- Initial draft implementation