[CWEP002] properly handle serialization of computed attributes Until this cset, only serialization of computed attribute's formula was handled (thanks to the relation definition's properties dict). We now: * properly serialize/deserialize attribute's formula * test first if the database has been migrated and contains related column or not Related to #3546717

add_relation_type('CWComputedRType')
add_attribute('CWAttribute', 'formula')