Separation of plotting and analysis functions

[leloup314]

Like in the generate_pixel_mask and cluster_hits function of hit_analysis, plotting is done separately from the analysis, with an independent plotting function from plot_utils. I guess this should be the general approach of handling plotting since it is much more conveinient to have separate functions in case one needs to plot something externally. Maybe we can implement this in the future for all functions of the analysis.

[DavidLP]

Yes, this is the way to do it. Right now there might be still some analysis parts in the plotting functions, since the separation of analysis and plotting was introduced when the code was already rather feature complete. Fell free to change that.