Calculate yield, P75, P90 from power

[SchmJo]

Foxes by default calculates power per turbine and state, invoking the power curve. It would be handy to have a turbine model that outputs yield in GWh, and also P75, P90 values, cf. model in flappy.

[SchmJo]

I wonder if we need this functionality as a TurbineModel, or rather as an output? Would of course be nice if the farm_results_eval output could run over it in the end, so it needs to be part of the farm calc results Dataset. But the additional variables like Yield, P90, P75 will probably not be needed during the main calculation

[MellBeli]

Created branch 14_yield_output to address this in farm_results_eval. Deleting branch 14_yield_model.

[SchmJo]

Done with this commit