Function follow_heating::brain::modes::working_temp::merge_hprt_into_fhxia

fn merge_hprt_into_fhxia(fhxia: f32, hprt: f32) -> f32

Gradually switch from using forecast HXI to using HPRT as temperatures get higher This may result in higher or lower circulation temps, but either way it aligns the top end of the range with the hard heatpump limit of 55deg HPRT.