Currently bonds cannot be created automatically from inside a file, 
using either the geometric 
Length or the topological 
Delaunay 
methods. Each bond object contains thus a complete description of one bond only.
To specify the atoms forming the bond, the bond parameters 
parent1, 
parent2 must point to the atom 
id attributes, which are
different for each atom.
Example: <bond ... parent1="C1" parent2="C2"/> (no default)
Allowed values: alphanumeric expression representing a unique atom id
with less than 20 characters (defined in GAMGI_ENGINE_TOKEN) (required)