reshape code so that the figures can be re-used in other modules (class based...

reshape code so that the figures can be re-used in other modules (class based plotting, add input loading function to read inputs files), split the code in different files (utils.py for utilities, figure.py for plotting related classes, commands.py for click commands). Change some click command defaults (nbins:  instead of 20, tablefmt:  instead of ), add click functionalities for a greater control of the plots (e.g. min/max axes values) and a nicer default visual. Fix the documentation and modify user guide according to the changes.