check existence of source terms before adding them to the rhs in the callbacks
check existence of source terms before adding them to the rhs in the callbacks

jrper/fluidity,rjferrier/fluidity,jjo31/ATHAM-Fluidity,jjo31/ATHAM-Fluidity,iakovos-panourgias/fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,rjferrier/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,rjferrier/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,iakovos-panourgias/fluidity,iakovos-panourgias/fluidity,jrper/fluidity