[Phylobase-devl] change root node from NA to 0

[Peter Cowan]

Given that significant changes are still fair game, I'd like to  
propose changes the root edge from having NA as the ancestor to 0 as  
the ancestor.

I'd be willing to do the lifting for this one, and could probably get  
it done around the same time the unification gets finished up.

I found myself having to work around the NA issue in several places  
where 0 wouldn't have been a problem.  Does anyone have a case where  
NA is superior to 0 in the edge matrix?

Peter