Bridge rule for FixedComp / SupplyPipe update / LayerComp update

Bridge rule : These are surfaces or rules that do not change under
some complementary operations. E.g. a divider surface. This has been
added to FixedComp. A few uses have been added in.

LayerComp now tracks the number of layers. This involves a change to the
constructor and a number of locations have been updated.

SupplyPipe [ess model] has been improved and made slightly more
relistic.