Only report significant geometry changes

[lxbarth]

Right now changewithin flags any change to a building way. So if you add for instance a node on a straight way in a manner that does not change the actual shape of the way changewithin would still flag the change.

This leads to a lot of false positives for instance where user robgeb adds 3d details to buildings that actually don't have an impact on the building outline.

[lxbarth]

The show changes PR https://github.com/osmlab/changewithin/pull/22 takes a first step in this direction by retrieving the full building polygon. As a next step, we'd have to test whether any node changed on the building outline had an impact on the building shape. We'll be able to discard not all classes of false positive but many.