Draft: Resolve "Check, whether AnalyzePowerSpectrum_fx() and AnalyzePowerSpectrum_ivas_fx() could be harmonized"

Attempt to harmonize AnalyzePowerSpectrum_fx() AnalyzePowerSpectrum_ivas_fx() by taking

1. AnalyzePowerSpectrum_fx()
2. AnalyzePowerSpectrum_ivas_fx() as only function.

Nr 1. is faster much less computational load