graphs: generate trees

[jasongrout]

From Chris Godsil's wishlist (with final reply by Robert Miller):

> 
>>> A database of trees, or a generator for trees. I think it would be
>>> reasonably
>>> straightforward to generate rooted trees, and hence get trees. The
>>> generators
>>> would be more useful than the database. It is not impossible that
>>> something
>>> in the nauty package generates trees.
> If you look into the code that graphs(7) calls, you will notice that
> you can pass it any vertex-hereditary property, including being a
> tree. So in some sense, this is already implemented. However, this
> could be its own constructor, and more importantly, I know of a way to
> optimize the generation of trees to go much faster than it would with
> what I described above. Create a ticket, and assign it to me.
> 

Component: combinatorics

Issue created by migration from https://trac.sagemath.org/ticket/1309

[rlmill]

comment:3

There are still significant bugs from the patch from

#1361

that I am working on now. I discovered them while implementing this ticket, so the fixes will be in the patch for this.

[rlmill]

Attachment: trees.patch.gz

[sagetrac-mabshoff]

comment:5

Merged in 2.8.15.rc0.