graph/coloring: allow warm start for DsaturExact
graph/coloring: allow warm start for DsaturExact

There is an optimisation that is available for the Dsatur recursive search
that takes advantage of the trivial observation that the colouring of a
maximum clique in the graph must be consistent with the colouring of the
graph and that a maximum clique will therefore constrain the colouring of
the graph. This means that if the maximum clique is first trivially coloured
the search can continue from there. However, if there are multiple co-equal
maximum cliques it is possible for a clique to be chosen that will force the
search down a pessimal search path, so we choose the clique to start from as
the clique with the least constraining effect on the rest of the graph. The
optimisation has the effect of excluding large sections of the search tree
with a small amount of initial work to assess the maximum cliques.

This change chooses the maximum clique with the lowest degree into the rest
of the graph and assigns colours to all its nodes and progresses from there.

gonum/gonum,gonum/gonum,gonum/gonum