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 - 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.

Use mat instead of csv (tex and new ops members are not saved ion csv files).

This member keeps track of the transformations and operations on the
variables. The name of the variables is now constant.

Octave does not add the *.mat extension by default.

(cherry picked from commit f1fbd6f4)

(cherry picked from commit bb2064b6)

Do not use exist function, check the output of dir() instead.