Relicense Sire from GPLv3+ to LGPLv3+

[lohedges]

This PR relicenses Sire under the GNU Lesser General Public License, version 3 or (at your option) any later version. All copyright holders of non-trivial surviving code have agreed to the change in #486, hence this closes #486.

It has two parts: removing the code that forced Sire to be GPL, then the relicense itself.

1. Removing GPL-derived code

• GSL (GPLv3) removed. SireMaths::Matrix SVD, eigendecomposition and covariance now use Eigen (MPL-2.0). Complex is backed by std::complex<double>, the gamma functions use Boost.Math, and areEqual and the numeric constants are implemented directly. GSL is removed from CMake, pixi.toml and the conda recipe.
• regress (GPLv3) removed. The polynomial fit and quadrature in SireAnalysis::TIPMF are reimplemented with Eigen (column-pivoted QR) and a plain trapezoidal rule.
• MDAnalysis-derived bond-order heuristic removed from SireRDKit. The fallback now calls MDAnalysis (>= 2.8.0, LGPL) directly through a Python callback. If MDAnalysis is not installed, determine_bond_orders=False raises an error and a failed determineBondOrders() emits a warning.

Regression tests were written against the GSL/regress builds before each swap: tests/maths/test_matrix_decompositions.py, tests/maths/test_complex.py, tests/maths/test_gamma.py and tests/analysis/test_ti_pmf.py.

Behaviour changes to be aware of:

• For rank-deficient input, Matrix::svd() now returns fully orthonormal U/V (Eigen), where GSL returned zero columns. kabasch() already handles this case, so alignment results are unaffected.
• arccsch(z) now uses arcsinh(1/z). The previous GSL-copied documentation gave arcsin(1/z).

2. Relicense

• LICENSE (and recipes/sire/LICENSE): the LGPLv3 text followed by the GPLv3 text it builds on, with a short note explaining why both are present. Both sections are verbatim from gnu.org.
• Source headers: all 1,240 full GPL headers and the 2,023 generated wrapper notices are updated, along with the wrapper generator (create_wrappers.py). Headers now refer to the LICENSE file, and the old FSF postal address is replaced by the URL. This commit was made with clang-format skipped so that it contains only the header change.
• Metadata: setup.py and the generated conda recipe use LGPL-3.0-or-later. Also updated: the PR template, CONTRIBUTING.md, corelib/build/README, download_compile_sire.py, the sire app licence banner (which said GPLv2), OpenMMMD.__license__, and the --help text of the legacy scripts in wrapper/python/scripts.
• Docs: acknowledgements.rst (licence statement, compatibility notes, a new xdrfile/xtc_seek entry), contributing/index.rst and the changelog.

Third-party code found during the audit:

• SireIO/third_party/xtc_seek.{c,h} (MDAnalysis) were GPLv2+. Their headers are updated to match upstream, which MDAnalysis has relicensed to LGPLv2.1+.
• SireMaths/complex.h/.cpp carried doc comments copied from the GSL manual (GFDL with invariant sections). These are rewritten, as are the matching docstrings in wrapper/Maths/Complex.pypp.cpp.
• Sire.Tools.ap (ascii plotting) had no licence, either in the file or upstream. It is removed, along with the optional ascii plots in analyse_freenrg.
• SireMaths/linearap.cpp contained a commented-out verbatim copy of Jonker's "freeware" LAP.CPP. It is removed. The live implementation is Sire's own port of the Jonker-Volgenant algorithm and keeps its attribution.
• The legacy package_sire self-extracting installer (makeself.sh, which is GPLv2, and the rest of corelib/share/build) is removed, together with its CMake install rule. Nothing else uses it.

Everything else that is vendored is permissive: xdrfile and MersenneTwister (BSD), sse/avx/neon_mathfun and md5 (zlib-style), sse2neon (MIT) and eig3 (public domain). The Qt headers in wrapper/Qt/qtheaders are used under Qt's LGPLv2.1 option.

Not changed

• The "(GPL3)" in the citation abstract in README.rst and doc/source/index.rst, which quotes the published paper.
• emle-engine (GPLv2+) is an optional runtime import that Sire does not ship, so it does not affect Sire's licence. acknowledgements.rst now says this; the note can be dropped once emle-engine is released under MIT.
• The Python API in src/ is LGPLv3+ with the rest of the package. It will be relicensed to MIT in a separate PR.

This PR was made with the aid of Claude Code (Opus 5.5).

Testing

• Full rebuild (corelib and wrappers)
• Full test suite, in particular tests/maths, tests/analysis and tests/convert/test_rdkit.py
• Clean install (CI conda build) no longer contains share/Sire/build

Checklist

• I confirm that I have merged the latest version of devel into this branch before issuing this pull request (e.g. by running git pull origin devel): [y]
• I confirm that I have added a test for any new functionality in this pull request: [y]
• I confirm that I have added documentation (e.g. a new tutorial page or detailed guide) for any new functionality in this pull request: [y]
• I confirm that I have added a changelog entry to the changelog (we will add a link to this PR as part of the review): [y]
• I confirm that I have permission to release this code under the LGPLv3 license: [y]

[lohedges]

It looks like the version of MDAnalysis on Python 3.10 isn't new enough. I'll investigate to see if there's a workaround, or guard the test, or drop 3.10 support, which we'll soon do anyway.