Fix ordering issues with --thr flag

Key change is to fix the bug where the calls to imglob were reordering ICs during the thresholding step.

Could do with an independent test, and would be worth checking the imglob logic in other parts of the file too perhaps? Note that the naming convention is now slightly different as we make dr_stage2_${s}_thresh files available directly in the output directory.

@steve @janineb