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 and imported by esis. 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, sun-data/aastex#17,
sun-data/solar-dynamics-observatory#22, and
sun-data/interface-region-imaging-spectrograph#48.

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 holders' names to endorse derived
products.

Changes

• LICENSE, the standard BSD 3-Clause text, with the copyright line
  Copyright (c) 2024-2026, Roy T. Smart and Jacob D. Parker. Those are the two
  contributors to this repository, and 2024 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.

Heads up on the Ruff check

Running ruff check . locally with ruff 0.16.3 reports one pre-existing error
unrelated to this change, D100 undocumented-public-module in docs/conf.py.
This repository sets preview = true and selects the D and DOC rule sets,
so the effective rules move with the ruff release, and the last Ruff workflow
run here was in June. If the check fails on this pull request, that is why, and
it needs its own fix rather than being folded in here.

Worth knowing before merging

The copyright line names the authors 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 (d2ae951) to head (982b32f).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #53   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           19        19           
  Lines          840       840           
=========================================
  Hits           840       840           

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

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