extract strValueTransform from superContext

Name of transformation method starts with 'strValueTransform_'
and then have a 'pseudo-unit-name' starting with 'str', which can then be
referenced as a 'unit' in regex group names.
Output units of transform method can be specified as function attribute
'units'
    def strValueTransform_strDate(self.string)
        ....
    strValueTransform_strDate.units = 's'
would than be used, if you have
    (?P<time_run_date_start__strDate>...)
in your regex.