Add NOT flow gate to achieve currently impossible self-balancing setups.

I was trying to build part of the water district:
- When both water & brine are needed, both flow normally into the system.
- When only one is needed, the needed one flows and the other one is dumped back into the sea. This is essential to adapt for supply variations and avoid death spirals.
- When none are needed, nothing flows to prevent wasting steam.
I don't see how this can be done in the current system. If there was something like NOT flow gate, this would be easily achievable. For example fluid flows only when there is or is not fluid in the side pipe.

Priorities: for brine, if the pipe to consumers overflows, discharge into the sea. For water, first to the boiler, then to consumers.