Initialize each fields of the fields_desc dict

def do_init_fields(self, flist):
        """
        Initialize each fields of the fields_desc dict
        """
        for f in flist:
            self.default_fields[f.name] = copy.deepcopy(f.default)
            self.fieldtype[f.name] = f
            if f.holds_packets:
                self.packetfields.append(f)