add missing const values; use some using instead of typedef

[andreacassioli]

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change. (actually this PR superseeds successive_shortest_path_nonnegative_weights const bugfix #160)
• 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

It superseeds #160

Motivation

Some const qualifiers are missing. Not a big deal, but it make it clearer to have them in place. Notice that the named parameter interface cannot be changed without introducing a breaking change: the reason is that it defaults to use graph internal property map as output, thus the graph must be modifiable to extract that map.

Incidentally I change some typedef to using.

Testing

Existing tests should be fine

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]

Compiler-warning counts vs develop (auto-generated).
PR run 27845986552 vs develop run 27551679795 (cf1839a3dc).

Job	Baseline	After	Delta
macos (clang, 14)	46	46	0
macos (clang, 17)	41	41	0
macos (clang, 20)	41	41	0
ubuntu (clang-19, 14)	46	46	0
ubuntu (clang-19, 17)	46	46	0
ubuntu (clang-19, 20)	46	46	0
ubuntu (clang-19, 23)	46	46	0
ubuntu (gcc-14, 14)	28	28	0
ubuntu (gcc-14, 17)	28	28	0
ubuntu (gcc-14, 20)	28	28	0
ubuntu (gcc-14, 23)	28	28	0
windows_msvc_14_3 (msvc-14.3)	974	974	0

[Becheler]

Thank you @andreacassioli for the little cleanup!
I made a last scan, the CI is green, no warnings added, the changes look good, so I will merge it 😄