Creates and adds a SymbolValidity object to the Symbol.

        Parameters
        ----------
        validity_template : bValidity or bValidity-like
            a validity template.

def add_validator(self, val_template):
        """
        Creates and adds a SymbolValidity object to the Symbol.

        Parameters
        ----------
        validity_template : bValidity or bValidity-like
            a validity template.

        """
        validator = val_template.validator
        
        args = []
        for arg in SymbolValidity.argnames:
            if arg in val_template.__dict__.keys():
                args.append(getattr(val_template, arg))

        objs = object_session(self)
        qry = objs.query(func.max(SymbolValidity.vid).label('max_vid'))
        qry = qry.filter_by(symname = self.name)
        
        cur_vid = qry.one()[0]
        
        if cur_vid is None:
            next_vid = 0
        else:
            next_vid = cur_vid + 1   
            
       
        self.validity.append(SymbolValidity(self, next_vid, validator, *args))
        objs.commit()