All algorithms related to categories and relations in CMF

Category objects allow to define classification categories
in an ERP5 portal. For example, a document may be assigned a color
attribute (red, blue, green). Rather than assigning an attribute
with a pop-up menu (which is still a possibility), we can prefer
in certain cases to associate to the object a category. In this
example, the category will be named color/red, color/blue or color/green.