FNIRT warped_file, log_file output name as inval

[BenjaminMey]

Summary

FNIRT warped_file and log_file seem to be split off from the other outputs in order to keep a possible ".txt" extension. However, they are also treated differently from all others if the input is set, i.e. setting outputs[key] = inval instead of outputs[key] = os.path.abspath(inval).

As expected FNIRT writes the files to the current working directory with the desired names. So, if warped_file or log_file are set to anything instead of generated, the node crashes because the path does not exist.

https://github.com/nipy/nipype/blob/232ac23bfd48240906e7e629e98eec854bfdcb1c/nipype/interfaces/fsl/preprocess.py#L1163```