Extend PAD option to plot more than one cell

Extend the -p PAD option to support drawing the values for different pads in a single canvas. <br> This is done extending the flat_plot::draw_flat function to use the --sc. <br> Using the example (16)

bin/HexPlot -i examples/test_data_IV.txt -g geo/hex_positions_HPK_128ch_6inch_FNAL.txt -o examples/example.png --IV --select 118 -p PAD --ys 1e9

<br> We can plot now more than one PAD with

bin/HexPlot -i examples/test_data_IV.txt -g geo/hex_positions_HPK_128ch_6inch_FNAL.txt -o examples/example2.png --IV --sc 4-18:118 -p PAD --ys 1e9

<br> And if we include more than one file in input such as in the following example:

bin/HexPlot -i /eos/user/h/hgsensor/HGCAL_test_results/Results/HPK_6in_135ch_1105/HPK_6in_135ch_1105_IV.txt,/eos/user/h/hgsensor/HGCAL_test_results/Results/HPK_6in_135ch_1106/HPK_6in_135ch_1106_IV.txt -g geo/hex_positions_HPK_128ch_6inch_FNAL.txt -o examples/example3.pdf --IV --sc 4-18:118 -p PAD --ys 1e9 --noav

The PAD option will produce also a comparison of the mean and the median among different files.