src/visu/figure_maker.py: Add a stat parameter to perform statistical analysis on the metagene figure

src/visu/figure_maker.py: removing the creation of a tmp file to speed up the creation of a figure made on the same input files

src/visu/__main__.py src/visu/figure_maker.py: modification to create a figure with two different list of exons

 src/visu/figure_maker.py src/visu/__main__.py: bin0_norm -> norm parameter that can either be a file or a bin number or None