Clarify geographic vs non-geographic bbox values

[tschaub]

This updates the language about bbox to align with GeoJSON for geographic coordinates and give min/max examples for non-geographic coordinates.

Fixes #112.

(One thing to note - the GeoJSON spec is a bit misleading in saying that the bbox has values for "all axes of the most southwesterly point followed by all axes of the more northeasterly point." What it means is that the value for $i_0$ is the most westerly, the value for $i_1$ is the most southerly, the value for $i_n$ is the most easterly, and the value for $i_{n+1}$ is the most northerly. The values for all other items are the minima followed by the maxima. The way the spec reads, it sounds like the third item of a bbox for geometries with a z would be the z value for the most southwesterly point. Instead it should be the minimum z value for all coordinates. I'll bring this up on the GeoJSON list and submit an errata report if others agree.)

[jorisvandenbossche]

(One thing to note - the GeoJSON spec is a bit misleading in saying that the bbox has values for "all axes of the most southwesterly point followed by all axes of the more northeasterly point." ...

Yes, agreed with your analysis. I assume that for the z dimension it is actually min/max, and not the z of the most southwesterly / northeasterly points. But that's indeed not clear from the GeoJSON description.

[jorisvandenbossche]

Looks good!