:bug:Power balance for power plant fails by the amount of the ohmic heating

The power balance check for the complete plant fails by an amount exactly equal to the ohmic power.
Note that the ohmic power lies entirely inside the power plant boundary (see Power flows), so it would not be correct to add a new line to the table.
What's happened is more subtle!
This error may be significant for RING.

************************ Power balance for power plant *************************
 
Fusion power (MW)                                                         (p_fusion_total_mw)             1.61307221011971478e+03 OP 
Energy multiplication in blanket and shield (MW)                          (p_blkt_multiplication_mw)      3.21225499318591744e+02 OP 
Total (MW)                                                                (p_nuclear_total_mw)            1.93429770943830658e+03 OP 
 
Net electric (MW)                                                         (p_plant_electric_net_mw)       3.92906268677036223e+02 OP 
Heat rejected by main power conversion circuit (MW)                       (p_turbine_loss_mw)             1.31078142304912285e+03 OP 
Heat rejected by other circuits (secondary heat) (MW)                     (p_plant_secondary_heat_mw)     2.31217280084996474e+02 OP 
Total (MW)                                                                (p_plant_out_mw)                1.93490497181115552e+03 OP 
Power plant overall imbalance (MW)                                        (p_plant_in_mw - p_plant_out_mw)  -6.07262372848936138e-01 OP 

Ohmic heating (MW)                                                        (p_plasma_ohmic_mw)             6.07262372848846321e-01 OP 

Steps to reproduce

Branch: 4233-global-power-imbalance-output
Input: large_tokamak_eval.IN.DAT

Expected behaviour

All four checks should give exactly zero imbalance.

Additional context

See #4233.
@jonmaddock @chris-ashe