spm wants a two-item array here

Hey folks,

In the SPM GUI, there's a note about this field: "An 1-by-2 array must be entered."

In the data structure that the SPM GUI creates, we see a 1-by-2 array.

In the nipype interface, however, we have a simple float, and it is passed to SPM as such in the generated .m file.

This fixes that, and runs fine.