Add packaging support

[matthewfeickert]

Though umnn is on PyPI (https://pypi.org/project/UMNN/) it doesn't have matching packaging support in this repository, which as far as I can tell from inspecting the v1.70 sdist is the source of the PyPI distribution.

Supporting valid packaging would be important to support other packages that want to build off of umnn as well as package on conda-forge.

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Related issues:

• Consider packaging and distribution on conda-forge sbi-dev/sbi#1322
• Consider packaging and distribution on conda-forge sbi-dev/pyknos#39
• Consider packaging and distribution on conda-forge bayesiains/nflows#82

[matthewfeickert]

(Double posting between here and email). Thanks very much for the fast review, @AWehenkel! If it is possible to get a new patch release of UMNN (which would require bumping the corresponding version number in

UMNN/pyproject.toml

Line 7 in 9041bff

version = "1.70"

) on PyPI I can then get UMNN up on conda-forge.

As you're the project creator I would be happy to add you as a maintainer of the UMNN conda-forge feedstock, though with no expectation that you'd be responsible for reviewing any automated maintenance of it. I can fully appreciate though that having "yet one more thing" tied to your GitHub might not be desirable. :)