All other things being equal preserve vertex order

[oxinabox]

Right now we do not take into account the order of the vertices in the graph at all,
except through the force_order functions and the like, which add a hard constraint on them.

However, there is a good change that the order is semantically meaningful,
while being less crucial than minimizing crossing. And so we should try to preserve it.
E.g. they might be ordered alphabetically.
In cases where minimizing crossings is easy (like a tree) then this can dominate the layout.

We should be able to add a prerve order term to the objective, and give it a very small weight such that any crossing always dominates it. Weighting with 1/num_vertices^2 should do that.
The term for preserving order should be to go through and add a term for each layer summing how often something of lower index is before something of higher.

One significant effect of this change is that it will make the ordering_problem solution unique,
and so will mean that adding more solve time will never find a better arrangement by searching over the existing solutions.
So it is arguably breaking, though I think we can just deprecate the argument for max_time and not tag a breaking release and noone will care. It rarely found a better solution anyway -- by symmetry often the next best solution was to just invert the order which still has equal score on both optimization problems.