#359 Structures being lost when merging

[Marcus Ennis]

When two compounds which both have checked structures are merged, the structure(s) not chosen as the new default are lost. They should be retained as alternative structures (as was the case once upon a time). I noticed this today when merging 17631 with 61299 and retaining the structure for 17631 as the new default. The structure originally present for 61299 has been lost completely.