replace fsl.IsotropicSmooth by Nilearn's smooth

[alexsavio]

# TODO in fmri.warp (and other places).
# I already tried this, but there is something that FSL does that Nilearn doesn't:
# When I look at the results in fslview the contrast is better with FSL,
# with Nilearn the image is very dark.

# smooth = setup_node(Function(function=smooth_img,
#                              input_names=["in_file", "fwhm"],
#                              output_names=["out_file"],
#                              imports=['from pypes.interfaces.nilearn import ni2file']),
#                      name="smooth_fmri")
# smooth.inputs.fwhm = get_config_setting('fmri_smooth.fwhm', default=8)
# smooth.inputs.out_file = "smooth_{}.nii.gz".format(wf_name)