[tei-council] stemma codicum in GD

[Lou Burnard]

I've now done my best to add a section on using <eLeaf> to represent a 
stemma codicum into chapter GD. I stole (but mangled) some of David's 
elegant prose, and all of his example; I also tried to keep in step with 
the subsequent discussion on this list at the beginning of this month. 
However, as you will see, I have made a few changes as follows.
- I recommend using <eLeaf> for terminal nodes rather than <eTree>
- I recommend using <ptr> rather than either a new <contaminates>  
element or the <relation> element.
Happy to defend these decisions if there is general consternation about 
them, but they seem to make good sense to me.