Add inspire categories.

        :param subject_terms: user categories for the current document.
        :type subject_terms: list

        :param source: source for the given categories.
        :type source: string

def add_inspire_categories(self, subject_terms, source=None):
        """Add inspire categories.

        :param subject_terms: user categories for the current document.
        :type subject_terms: list

        :param source: source for the given categories.
        :type source: string
        """
        for category in subject_terms:
            category_dict = self._sourced_dict(
                source,
                term=category,
            )
            self._append_to('inspire_categories', category_dict)