Set value of anchor attribute on ``<a:tcPr>`` child element

def anchor(self, anchor_enum_idx):
        """
        Set value of anchor attribute on ``<a:tcPr>`` child element
        """
        if anchor_enum_idx is None and self.tcPr is None:
            return
        tcPr = self.get_or_add_tcPr()
        tcPr.anchor = anchor_enum_idx