Create a sub state with only the gene passed in arguments.

        Example
        -------

        >>> state.sub_state_by_gene_name('operon')
        {operon: 2}
        >>> state.sub_state_by_gene_name('mucuB')
        {mucuB: 0}

def sub_state_by_gene_name(self, *gene_names: str) -> 'State':
        """
        Create a sub state with only the gene passed in arguments.

        Example
        -------

        >>> state.sub_state_by_gene_name('operon')
        {operon: 2}
        >>> state.sub_state_by_gene_name('mucuB')
        {mucuB: 0}
        
        """
        return State({gene: state for gene, state in self.items() if gene.name in gene_names})