Headless N-linked CHO

Now you can build N-linked carbohydrate without a gui:

$ coot --no-graphics --script headless-cho.scm

where headless-cho.scm is:

(let ((imol (read-pdb "pdb2qc1-no-cho.ent")
   (make-and-draw-map "2qc1_map.mtz" "FWT" "PHWT" "" 0 0)
   (add-linked-residue-tree 0 (list "B" 141 "") oligomannose-tree) ;; add to this ASN
   (write-pdb imol "with-cho-added.pdb"))