Type check SpatialExtent bboxes parameter

[CloudNiner]

It's easy to accidentally forget to pass SpatialExtent(bboxes) as a list[list[float]] and just pass a single dimensional bbox. Here's two examples of this happening, #197 and azavea/noaa-flood-mapping#105. This leads to writing a STAC Catalog incompatible with the spec.

Two potential improvements?

1. Throw a TypeError or some equivalent exception when the incorrect type is passed
2. pystac could implicitly wrap a passed list[float] to list[list[float]]