Generic function for 1/f noise

[zen-juen]

Would be good to have a generic signal_1f function to measure 1/f noise in particular for eeg and ecg signals (many papers have been published on this e.g., here)

To-do:

• [ ] Dissociate the functionalities for our existing fractal_psdslope - slope itself should be computed in signal_1f and this can then be embedded into fractal_psdslope which converts to an estimate of fractal dimension
• [ ] Additional parameter considerations for computing 1/f (ref FOOOF tool)