Gets the XML field object of the given name.

def get_field(self, name):
        """
        Gets the XML field object of the given name.
        """
        # Quicker in case the exact name was used.
        field = self._all_fields.get(name)
        if field is not None:
            return field

        for field_name, field in self._all_fields.items():
            if self._sanitize_field_name(name) == self._sanitize_field_name(field_name):
                return field