These routines will extract the average counts/sec of all 64 sensors.
The input can be roadmap, index, or observing log and it will
get the proper structure tag and decompress it properly to
It is possible to get a string defining the
channel selected by using the title keyword option.
IDL> y = gt_sum_l(roadmap)
IDL> y = gt_sum_l(w_h, title=title)