A helper for eval expressions on ports

[jonathanjfshaw]

I'm about to PR a node in timeflux_dsp that uses arbitrary rules to classify data. The rules are eval'd as expressions using ports, in the way the Expression node does.

This eval logic from Expression seems like it might be useful in quite a few contexts. It's something like this:

    def _eval_expression(self, expression):
        ports = [port_name for port_name, _, _
                                in self.iterate('i_*')
                                if port_name in expression]
        _local_dict = {port_name: self.ports.get(port_name).data
                       for port_name in ports}
        if np.any([data is None for data in _local_dict.values()]):
            return
        calculated = pd.eval(expr=expression,
                              local_dict=_local_dict,
                              **self._kwargs)
        for port_name in ports:
            self.o.meta.update(self.ports.get(port_name).meta)
        return calculated

Should it be a helper in helpers/ports.py, or should the Expression node be refactored so that other nodes can extend it more easily and inherit this method from it?