Update fmri_data.m to avoid rezip

[jcf2]

Rezip to get back to .gz can be avoided in fmri_data if have original .gz (which we always should).

Fixes #60

[Michael-Sun]

system call to gunzip (unsafe) replaced with native matlab gzip() which is safe and non-destructive.

[jcf2]

To clarify, note that the use of native MATLAB 'gunzip' predates this change (was probably introduced ~5 years ago?). The issue (#60) fixed here is basically that when that switch was made, the "cleanup" should have changed but didn't. System 'gunzip' by default deletes the original .nii.gz, so the "cleanup" step up until now has included recreating the .nii.gz to counter that... when using MATLAB 'gunzip', however, the original .nii.gz is guaranteed to remain, so the only cleanup really needed is to remove the unpacked .nii file(s).