Added XOR to mkmask and source_file
Added XOR to mkmask and source_file

mri_surf2vol can either be run using a source file OR without a
source_file if the user wants to create a binary mask. Added XOR
parameter to take this behavior into account

sgiavasis/nipype,carolFrohlich/nipype,carolFrohlich/nipype,FCP-INDI/nipype,FCP-INDI/nipype,sgiavasis/nipype,mick-d/nipype,mick-d/nipype,mick-d/nipype,mick-d/nipype,sgiavasis/nipype,sgiavasis/nipype,FCP-INDI/nipype,carolFrohlich/nipype,carolFrohlich/nipype,FCP-INDI/nipype