enh: allow a single mask for fixed image
enh: allow a single mask for fixed image

mick-d/nipype,glatard/nipype,FCP-INDI/nipype,grlee77/nipype,carolFrohlich/nipype,JohnGriffiths/nipype,sgiavasis/nipype,Leoniela/nipype,arokem/nipype,carlohamalainen/nipype,rameshvs/nipype,blakedewey/nipype,gerddie/nipype,glatard/nipype,carolFrohlich/nipype,dgellis90/nipype,JohnGriffiths/nipype,mick-d/nipype,dmordom/nipype,mick-d/nipype_source,sgiavasis/nipype,fprados/nipype,mick-d/nipype,iglpdc/nipype,rameshvs/nipype,pearsonlab/nipype,blakedewey/nipype,JohnGriffiths/nipype,blakedewey/nipype,wanderine/nipype,gerddie/nipype,mick-d/nipype,iglpdc/nipype,glatard/nipype,mick-d/nipype_source,Leoniela/nipype,pearsonlab/nipype,sgiavasis/nipype,arokem/nipype,pearsonlab/nipype,Leoniela/nipype,carolFrohlich/nipype,dmordom/nipype,JohnGriffiths/nipype,grlee77/nipype,fprados/nipype,blakedewey/nipype,wanderine/nipype,gerddie/nipype,sgiavasis/nipype,glatard/nipype,FCP-INDI/nipype,FCP-INDI/nipype,iglpdc/nipype,gerddie/nipype,iglpdc/nipype,carolFrohlich/nipype,arokem/nipype,wanderine/nipype,arokem/nipype,dmordom/nipype,carlohamalainen/nipype,wanderine/nipype,dgellis90/nipype,carlohamalainen/nipype,dgellis90/nipype,FCP-INDI/nipype,grlee77/nipype,rameshvs/nipype,grlee77/nipype,mick-d/nipype_source,pearsonlab/nipype,rameshvs/nipype,dgellis90/nipype,fprados/nipype