'Parse and validate the input value, but don\'t create or update
any records. This will be called by S3CRUD.validate to validate
inline-form values.
To be implemented in subclass.
@param value: the value from the form
@returns: tuple (values, error) with values being the parsed
value dict, and error any validation errors'

def validate(self, value):

    values = self.parse(value)
    return (values, None)
