The IEnvelope class is a three-dimensional, axis-aligned box. This is a helper type for Geometries, and does not itself inherit from IGeometry. This type is typically used to record the "extents" of a shape, also known in 2 dimensions as an MBR ("Minimum Bounding Rectangle"). The box may be initially empty, meaning that no dimensions have an assigned value. Each individual extent may be unassigned, in which case using its accessor will return numeric_limits::quiet_NaN().
