ConjugateGradientSolver works with Eigen types

[evaleev]

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[Copilot]

Pull request overview

This PR extends the solver utilities so ConjugateGradientSolver (and related solver infrastructure) can operate directly on Eigen vector/matrix types by ensuring the required freestanding linear-algebra helpers are discoverable via ADL and by avoiding hard member-function requirements in generic tile interfaces.

Changes:

• Add Eigen-specific specializations/tests to validate ConjugateGradientSolver<Eigen::VectorXd, ...> behavior.
• Constrain several non-intrusive tile interface helpers (abs_min/abs_max/dot/inner_product, clone) with SFINAE so they don’t break ADL-based resolution for non-TiledArray types (e.g., Eigen).
• Add missing Eigen freestanding adapters (clone, volume, abs_min, abs_max) and switch solver numeric type deduction to TiledArray::detail::numeric_t.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/solvers.cpp	Adds an Eigen-based conjugate gradient unit test and supporting helpers.
src/TiledArray/tile_op/tile_interface.h	Adds SFINAE constraints so these helpers only participate when the type has the corresponding member function.
src/TiledArray/tile_interface/clone.h	Adds SFINAE constraint to the member-based TiledArray::clone to avoid blocking ADL for non-TiledArray types.
src/TiledArray/math/solvers/diis.h	Uses numeric_t<D> for value type deduction (Eigen compatibility) and includes type traits.
src/TiledArray/math/solvers/conjgrad.h	Uses numeric_t<D>, updates docs to abs_min/abs_max, and makes DIIS usage compile-time gated.
src/TiledArray/math/linalg/basic.h	Adds Eigen freestanding adapters (clone, volume, abs_min, abs_max) needed by solver requirements.

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[Copilot]

use_diis is a compile-time false, but DIIS<D> diis is still constructed unconditionally (including allocating B_ in DIIS::init()), even though it can never be used. Consider constructing DIIS only inside an if constexpr (use_diis) block (or via a lazily-emplaced optional) so the disabled DIIS path has zero runtime overhead.