[Bug] Type error in afni's CenterMass

```Python
from nipype.interfaces import afni
a = afni.CenterMass()
a.inputs.in_file = 'anat.nii.gz'
a.inputs.cm_file = 'cm.txt'
a.run()
```
throws an error because single line file `cm.txt` is not loaded in an array of length one
```
TypeError: 'numpy.float64' object is not iterable
```