'Validator method for the input field, used to extract the
data from the input field and prepare them for further
processing by the accept()-method. To be implemented in
subclass and set as requires=self.parse for the input field
in the resolve()-method of this form element.
@param value: the value returned from the input field
@return: tuple of (value, error) where value is the
pre-processed field value and error an error
message in case of invalid data, or None.'

def parse(self, value):

    return (value, None)
