Clarify and decide what the unknown dimension does in `geo-traits`

[frewsxcv]

There was some discussion in Discord, but I also want to track this issue in GitHub. Should this block a geo-traits 0.2.0 release?

[kylebarron]

Perhaps we should make a little progress here before 0.2.0

[frewsxcv]

Relevant discussion: https://github.com/georust/geojson/pull/245#issuecomment-2466771255

[kylebarron]

Consolidating discussion here from https://github.com/georust/geojson/pull/245

I would prefer not having dim return Option, because I think it's unreasonable to ask every consumer to check for a null dimension.

In the GeoJSON case, it would only apply to empty geometries. How wrong would it be to pass on Unknown(2), or something like that. The GeoJSON geometry is empty, so it could have two dimensions. Maybe lying in a way that could be true in very rare circumstances is better than making every reader consider None

[frewsxcv]

For the time being, that seems alright to me. In which case, that wouldn't require any changes to the current dimensions implementation, right?

[kylebarron]

Correct I believe, and then we can continue the discussion after 0.2 is released