VertexRoutingInfo continue to Support None Continuous keys and mask

[Christian-B]

Found while looking at #438

The VertexRoutingInfo has a list of BaseKeyAndMask

Is this desirable or is it better to reduce to a single continuous BaseKeyAndMask

[andrewgait]

I think on looking at what's coming in the external devices / fpga branch, there is new functionality which assumes this is a list. But I think it's worth keeping this issue open at least until those branches get reviewed.