Add a BSD 3-Clause license

[roytsmart]

This repository has no license, which means the default of all rights reserved
applies: nobody has permission to use, modify, or redistribute it, even though
it is published on PyPI. This is the same change merged as
sun-data/regridding#54, sun-data/named-arrays#203, sun-data/colorsynth#11,
sun-data/ndfilters#28, sun-data/optika#199, and sun-data/ctis#24.

BSD 3-Clause matches the rest of the organization. It also satisfies NASA
SPD-41a, which asks that software developed with Science Mission Directorate
funding be released under a permissive license, and its third clause forbids
using the copyright holder's name to endorse derived products.

Changes

• LICENSE, the standard BSD 3-Clause text, with the copyright line
  Copyright (c) 2023-2026, Roy T. Smart. You are the only contributor to this
  repository, and 2023 is when it was created.
• pyproject.toml declares license = "BSD-3-Clause" and
  license-files = ["LICENSE"], the PEP 639 form.

Verified by building a wheel from this branch: the metadata is
Metadata-Version: 2.4 with License-Expression: BSD-3-Clause and
License-File: LICENSE, and the license is packaged at
dist-info/licenses/LICENSE, so PyPI will display it on the next release.
There is no License :: classifier to conflict with the expression.

Worth knowing before merging

The copyright line names the author rather than Montana State University. MSU's
faculty handbook gives authors ownership of copyrightable works other than
commissioned courseware, which is the basis for that, but the university has
not been asked, and if it ever asserts ownership the line is an ordinary commit
to correct. Releasing permissively is the part that does not undo: copies
distributed under these terms stay licensed under them.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a4b6eb6) to head (827c4bf).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #17   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          702       702           
=========================================
  Hits           702       702           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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