expand helix boundaries based on offsets after mirroring strands

[dave-doty]

On this design:

If you select reflect the strand horizontally, it goes outside the bounds of helix 1:

This should be checked and prevented in check_mirror_strands_legal_middleware.

UPDATE: I no longer want to prevent this reflection. Now I think it's a better idea to simply expand the necessary helices, in accordance with eventually implementing #579.

[dave-doty]

@abhishekgokhale Note this is related to issue #579. It can be implemented separately if that's easier, but keep it in mind when working on #579.