fsl.MotionOutliers output spec no longer requires out_file to exist

[beausievers]

fsl.MotionOutliers output spec no longer requires out_file to exist. fsl_motion_outliers does not create an output file when no timepoints are above the threshold, and in this situation fsl.MotionOutliers would throw a FileNotFoundError. This pull request fixes that immediate problem.

[coveralls]

Coverage remained the same at 72.538% when pulling 95b4fc8 on beausievers:master into 4b392d5 on nipy:master.

[satra]

@oesteban, @chrisfilo - we need to come up with a reasonable solution before this release. turns out the file is not created when the threshold does not result in any outliers. i was going to write a list outputs for this interface, but do you think we could have the following pattern.

if metadata exists does not exist
   and file pattern created by name_template does not match an existing file
then keep output undefined

[mgxd]

@satra @oesteban has this been solved or should we merge this? related to #1427

[effigies]

exists=True is a blight in output specs. I'm merging regardless.