- New class member tags

 - Member tags is a structure, initialized to empty.

 - Each user-defined field of this structure must be a vobs(o)*1 cell
   array (each element is associated to a variable).

 - To add a new tag name:

      >> o.tag('type')

 - To associate a tag value to a variable:

      >> o.tag('type', 'Consumption', 'Flow')

   the first argument is the tag name, the second argument is the name
   of the variable, the last argument is the value of the tag.