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 across the rest of the
organization: sun-data/regridding#54, sun-data/named-arrays#203,
sun-data/colorsynth#11, sun-data/ndfilters#28, sun-data/optika#199,
sun-data/ctis#24, and sun-data/aastex#17.

BSD 3-Clause matches numpy and astropy, which this package sits on top of.
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, so PyPI will display it on the next release. There is
no License :: classifier to conflict with the expression, and ruff check .
passes.

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 (b09a55b) to head (9748ea4).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #48   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines          561       561           
=========================================
  Hits           561       561           

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

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