Bug: Cannot merge a list of 600 files

[AlanKuurstra]

After registering all images to the avg during motion correction, we desire to use fsl merge to recombine the motion corrected images back into a 4d file. However, a command on the shell cannot have an input list of 600 and so the merge command fails:

OSError: [Errno 7] Argument list too long: '/bin/sh'

a solution is to perform the merge ourselves in python