Default value of boxplot percentile should be None

In plot.py, the percentile could be either None or a float between 0 and 1, but the type defined in click option is float. So None is not a valid entry in the command line (--percentile None). In this case, the default for option --percentile should be None so that it is possible to change to a float later. When the default is a float like 0.5, then if we want to take the whole scores to make boxplot, then entry --percentile None will bring an Error.

I have this command updated in branch change_boxplot_percentile_default, but it cannot pass the CI/CD because the name updates in bob.bio.base are not updated here. I will take care of it if necessary.