Code updates

[TomDonoghue]

Updates / additions to make to code:

• plots: for visualizing model components (plot_peak_fits and plot_aperiodic_fits), it would be nice to support the use case of plotting the mean + shading (sem or std). This is perhaps better for comparing groups. Note that this might need to be a new func. (PR'd in [ENH] - Add options to shade param reconstructions #290)
• add average_model_values helper function. We currently average parameters (in average_fg), but a useful alternative would be to average the model reconstructions (PR'd in [ENH] - Add average_reconstructions function #289)
• add interpolate_spectra function (see [DOC] - Add a line noise example #203 (comment); done in [ENH] - Add interpolate_spectra function #288)

[TomDonoghue]

This is all done / in progress - so closing this now and discussion can move to individual PRs.