docs: credit the L test, FFT indexing, TORO, Niggli, peakfinder8 and SparseCCL

Six methods the pages name or describe carried no citation: Padilla & Yeates
(the L test), Steller, Bolotovsky & Rossmann (the projection/FFT autoindexing
MOSFLM implements), TORO (what ffbidx implements), Krivy & Gruber and the
ITA lattice-character table (the reduction and Bravais assignment), Cheetah's
peakfinder8 (the per-ring background statistics of the adaptive finder) and
Hennequin et al.'s SparseCCL (already credited to traccc, now also to its
authors). Each gets its ACKNOWLEDGEMENT.md paragraph, a References entry in
CPU_DATA_ANALYSIS.md, and a one-line credit at the algorithm. The
Sheriff & Hendrickson / Popov & Bourenkov entry is re-scoped so each claim
sits on the paper that supports it - P&B 2003 is titled, and credited for the
sigma-aware anisotropy estimation its statistic modelling contains, not for
the tensor and its constraints. All DOIs verified against the publishers;
the SparseCCL DOI resolves to IEEE document 9049184 (IEEE blocks content
scraping, so verified by the resolved document id plus two independent
sources).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EFEJG6WBQv8th4UJFNe53N

image_analysis/spot_finding/StrongPixelSet.cpp

@@ -4,6 +4,7 @@
 // SparseCCL code taken from https://github.com/acts-project/traccc/blob/main/core/include/traccc/clusterization/detail/sparse_ccl.hpp
 // (c) 2021-2022 CERN for the benefit of the ACTS project
 // Mozilla Public License Version 2.0
+// The algorithm: Hennequin, Couturier, Gligorov & Lacassagne (2019) DASIP 2019, 65-70 (SparseCCL)
 //
 // The union-find and the two-scan structure are theirs. How a pixel's earlier neighbours are FOUND
 // is not: see sparseccl below.