The following page in the documentation (and the sage docstring for this method)
https://doc.sagemath.org/html/en/reference/graphs/sage/graphs/graph.html
states chromatic_number uses the less efficient DLX algorithm by default, as opposed to the MILP algorithm that is sometimes more efficient.

Here, we add the 'parallel' algorithm that executes all algorithms in parallel and returns the answer as soon as one of the algorithm terminates.

Related ticket: #12378

Component: graph theory

Keywords: chromatic_number

Author: Joal Heagney, David Coudert

Branch/Commit: public/graphs/12379_parallel @ e05de25

Issue created by migration from https://trac.sagemath.org/ticket/12379