convert ID columns from ilwd:char to int_8s

• changes are confined to string formatting in XML I/O routines, there is no binary API change associated with this (IDs have always been stored internally as integers, not ilwd:char strings).
• this has been announced a number of times, especially to the compact objects working group, and no objections have been raised. the gstlal team is ready to proceed with this conversion at this time.