Strategy for determining whether an alignment has changed in a new version

[tdudgeon]

When a new version of data is created the alignment for an existing structure may or may not change, depending on:

1. whether the structure itself has changed
2. whether the site it is being aligned to has changed

We already have a mechanism for determining the first of these, but not for the second. We need a strategy for being able to determine this. @ConorFWild has some ideas here that need exploring.