fix global signal extraction
fix global signal extraction

shoshber/fmriprep,poldracklab/preprocessing-workflow,poldracklab/preprocessing-workflow,oesteban/fmriprep,shoshber/fmriprep,poldracklab/fmriprep,poldracklab/fmriprep,chrisfilo/fmriprep,oesteban/preprocessing-workflow,oesteban/fmriprep,poldracklab/fmriprep,oesteban/preprocessing-workflow,oesteban/fmriprep