Removing many dependencies to Boost.MPL

[Becheler]

Reducing dependencies deadweights.

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

Migrates Boost.Graph's internal metaprogramming from Boost.MPL to the standard <type_traits>.

• Removed dead boost/mpl includes (included but unused)

  • or.hpp in subgraph, void.hpp in graph_traits, print.hpp + if.hpp in CSR
  • if.hpp in directed_graph / undirected_graph / random_layout / detail/indexed_properties /detail/compressed_sparse_row_struct / graph_mutability_traits
  • bool.hpp in graphml / planar_detail/face_iterators
  • and.hpp + not.hpp in properties

• Converted internal metafunctions off MPL (resolved type identical)

  • mpl::if_<C,A,B>::type to std::conditional<C::value,A,B>::type : adjacency_matrix, subgraph, properties, plod_generator, random, edge_list, labeled_graph, adjacency_list, detail/adjacency_list
  • mpl::if_c<…> to std::conditional<…> : betweenness_centrality, property_iter_range
  • mpl::and_<A, mpl::not_<B>>::type to std::integral_constant<bool, A::value && !B::value> : adjacency_list, detail/adjacency_list
  • mpl::not_<is_same<X,void>>::value to !is_same<X,void>::value : graph_concepts
  • mpl::eval_if<C,F1,F2>::type to std::conditional<C::value,F1,F2>::type::type (lazy branch instantiation preserved) : labeled_graph
  • mpl::bool_<B> to std::integral_constant<bool,B> : labeled_graph (is_default)
  • mpl::int_<N> to std::integral_constant<int,N> : internal tag dispatch in vf2_sub_graph_iso
  • internal true_/false_ dispatch tags to std::true_type/std::false_type : visitors, edge_list (is_random), adjacency_matrix, detail/is_distributed_selector
  • boost/mpl/*.hpp to <type_traits>

Motivation

MPL is an old dependency with long build times. It has been superseded by modern C++ standards and MP11.
This refactor switches to std and where possible.

Testing

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 29508019477 vs develop run 29503421582 (e7fd958338).

Header-inclusion weights (graph files pulling each direct dependency in):

Dependency	develop	PR	Δ
mpl	30	9	-21

Transitive Boost modules: 69 → 69 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 29508018791 vs develop run 29503421610 (e7fd958338).

Job	Baseline	After	Delta
macos (clang, 14)	4	4	0
macos (clang, 17)	4	4	0
macos (clang, 20)	4	4	0
ubuntu (clang-19, 14)	4	4	0
ubuntu (clang-19, 17)	4	4	0
ubuntu (clang-19, 20)	4	4	0
ubuntu (clang-19, 23)	4	4	0
ubuntu (gcc-14, 14)	10	10	0
ubuntu (gcc-14, 17)	10	10	0
ubuntu (gcc-14, 20)	10	10	0
ubuntu (gcc-14, 23)	10	10	0
windows_msvc_14_3 (msvc-14.3)	971	971	0