you specify a method to call on each value to render them.
The method must accept the following parameters :
value = the value to display
cell_name = the name of the cell in pdfform.
It should be backward compatible (if not, contact me)


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@3324 20353a03-c40f-0410-a6d1-a30d3c3de9de