Test data

[htwangtw]

FSL interfaces need some different type of test data, and they are not always a nifti file.
It would be useful to identify some light weight test data that already exist in nipype and migrate them here.

cc @effigies if you have a better idea.

[htwangtw]

Link to all test data in nipype
Welp this is a lot of things - I have a feeling that the easiest way to handle it to do it case by case.