adding optional arg into grad_ele_dg sub
adding optional arg into grad_ele_dg sub

jjo31/ATHAM-Fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,jrper/fluidity