Compute the bend frequency

[astrolamb]

Is your feature request related to a problem? Please describe. Input spectrum models have a peak frequency. It'd be nice to have a separate method to compute it for given spectral parameters

Describe the solution you'd like Add a separate method in the input models to compute f0

Describe alternatives you've considered I can just copy the code out of model.py and add my own f0 function for each input model. But it'd be nice to have it as a formal part of each input spectrum

Additional context I want to see where the peak frequency compares to the PTA band