ENH: Allow only NIfTI or CIfTI inputs to be cleaned

The pyfix cleaning stage (fix -a ...) supports a new option which allows only the NIfTI or CIfTI data (filtered_func_data.nii.gz or Atlas.dtseries.nii) to be cleaned (the default is to clean both).

This new option (-o or --only) can be used like so:

fix -a labels.txt melodic.ica>          # clean both NIfTI and CIfTI data if present
fix -a labels.txt melodic.ica> -o nifti # only clean NIfTI data (filtered_func_data.nii.gz)
fix -a labels.txt melodic.ica> -o cifti # only clean CIfTI data (Atlas.dtseries.nii)