Use igraph instead of NetworkX

[mdbrnowski]

The igraph library does not support planarity checks, so we leave that to networkx for now. All other properties are checked with igraph.

Thanks to this change, creating the maximal database ($n \leq 9$) now takes 2 minutes (instead of 25 minutes).

Resolve #21.

[Copilot]

Pull request overview

This PR improves database creation performance by switching most graph property computations from NetworkX to igraph, while keeping NetworkX only for planarity checking (since igraph doesn’t support it yet). It addresses Issue #21 by significantly reducing the time needed to generate the maximal database (up to n < 9).

Changes:

• Replaced NetworkX-based Hamiltonian and degree utilities with igraph equivalents and added an igraph-based Eulerian check.
• Updated database creation to convert graph6 → NetworkX → igraph, computing most properties with igraph and planarity with NetworkX.
• Updated docs and dependencies to include igraph and reflect new runtime expectations.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/graphreveal/db_creator/util.py	Migrates helper algorithms from NetworkX to igraph and adds is_eulerian.
src/graphreveal/db_creator/create.py	Uses igraph for most properties during DB creation, retaining NetworkX only for planarity.
README.md	Updates expected database creation timings and wording.
pyproject.toml	Adds igraph as a project dependency.

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