add test and doc delete_(single_)ode

ode.pl

@@ -830,6 +830,10 @@ put_ode_into_system :-
   ).
 
 
+%! delete_single_ode(+Name:name) is det
+%
+% deletes the ODE associated with variable Name
+% necessitates a current ODE system
 delete_single_ode(Name) :-
   get_current_ode_system(Id),
   delete_item([parent: Id, kind: ode, key: Name]).

ode.plt

@@ -49,6 +49,8 @@ test(
   new_ode_system,
   add_ode(da/dt = -a),
   add_ode(db/dt = a),
+  add_ode(dc/dt = 1),
+  delete_ode([c]),
   import_reactions_from_ode_system,
   all_items([kind: reaction], Reactions).