Consider nfm publish to make a niflow available for public retrieval

[effigies]

In the Python case, the publication of a package involves running cd package && python setup.py sdist bdist_wheel && twine upload dist/* or something similar. This is separate from updating versions, tagging and pushing to GitHub.

Each niflow can obviously set up its own processes, but we might want to think about what we can do to make the publication process a bit smoother by default.