spm.preproces.realign doesn't expect the use of write_which

[beOn]

This issue is secondary to #531.

After you apply the patch from #532, if you use one of the values for write_which that prevent the reslicing of all images (eg. [0, 1]), this part of Realign's _list_outputs fails, because it tries to add files to the output that have not in fact been created.

if self.inputs.jobtype == "write" or self.inputs.jobtype == "estwrite":
            if isinstance(self.inputs.in_files[0], list):
                first_image = self.inputs.in_files[0][0]
            else:
                first_image = self.inputs.in_files[0]

            outputs['mean_image'] = fname_presuffix(first_image, prefix='mean')
            outputs['realigned_files'] = []
            for imgf in filename_to_list(self.inputs.in_files):
                realigned_run = []
                if isinstance(imgf,list):
                    for inner_imgf in filename_to_list(imgf):
                        realigned_run.append(fname_presuffix(inner_imgf, prefix=self.inputs.out_prefix))
                else:
                    realigned_run = fname_presuffix(imgf, prefix=self.inputs.out_prefix)
                outputs['realigned_files'].append(realigned_run)

I will submit a pull request to address this shortly.