Modified Knee Fit

[ryanhammonds]

I have a modified knee model that could be added as an option to the aperiodic funcs. The modification adds a constant to the aperiodic knee model to fit the psd of spikes trains, allowing power to taper off to a constant value at high frequencies. It may be useful to add here at some point. The modification below also directly fits a knee frequency (mentioned in #224).

knee_fit = np.log10(10**b * (1 / (knee**(exp) + xs**(exp))) + c)