Get a TermCounts object if the user provides an annotation file, otherwise None.

def _get_tcntobj(goids, go2obj, **kws):
        """Get a TermCounts object if the user provides an annotation file, otherwise None."""
        # kws: gaf (gene2go taxid)
        if 'gaf' in kws or 'gene2go' in kws:
            # Get a reduced go2obj set for TermCounts
            _gosubdag = GoSubDag(goids, go2obj, rcntobj=False, prt=None)
            return get_tcntobj(_gosubdag.go2obj, **kws)