Change entries.py to support branch and taxonomy name

[alexgarel]

We want to edit a taxonomy with a branch name and a taxonomy name.

Change entries.py so that it defines a class (I propose TaxonomyGraph).

The class takes branch (branch name) and taxonomy name as parameters.

Current functions become methods of this class.

So that the caller can do:

taxonomy = TaxonomyGraph(branch_name, taxonomy_name)
taxonomy.get_nodes(...)

Modify those methods to add the branch_name and taxonomy_name labels. First is prefixed by b_, second is prefixed by t_