refactor : cols_desc is now simply cols and is passed as a kwoption of the loader __init__
	modified:   census/loader.py
Add kwoption specials to ValueMapper to handle special values in the future
	modified:   census/transformer.py
fully rewrite of BasicStatistic
modified:   census/stats.py
renamed regroup_unstack by count_value
	modified:   census/stats.py
Changed the way columns are described in data
	modified:   census/loader.py
	modified:   census/meta.py
Added the col description to stat plots
	modified:   census/stats.py
renamed count_values by count_values_per_target to avoid confusion with pandas
	modified:   census/stats.py
Manage zero value in raw data plotting when there are too numerous
	modified:   census/stats.py
renamed stepped by perct_col in method plot_num_stat
	modified:   census/stats.py