MassFlowRate is inappropriate in processes with volume balances (such as THM), where a VolumeFlowRate is output (m³/s instead of kg/s). That's confusing and easily leads to errors. Not sure what the status in all the other processes is ... We can either fix the names -- meaning that different processes have different flow specifiers (con) that indicate their physical dimension (pro). Or we choose general names: NodalFlowRate_p, NodalFlowRate_T, etc.