added check to ensure workflows are directed acyclic graphs
added check to ensure workflows are directed acyclic graphs

git-svn-id: 24f545668198cdd163a527378499f2123e59bf9f@1671 ead46cd0-7350-4e37-8683-fc4c6f79bf00

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