Auto-regulate FBR option for enriched blanket output.

Right now the only auto-regulate method for a FBR is for its steam output. In the case of 3x reactors, it would be very convenient to instead have them ramp their operation based on demand for enriched blanket. This would allow them to scale up and down based on space needs, or downstream 0x FBR needs which are set to the existing steam auto-regulation system.