Datacube `VerticalSpatialDimension` enforces optional `extent`

[fmigneault]

Contrary to Horizontal Spatial Raster Dimension Object, the Vertical Spatial Dimension Object does NOT require an extent.

However, given the following dependency:

pystac/pystac/extensions/datacube.py

Line 216 in 6e8eb22

class VerticalSpatialDimension(SpatialDimension):

And the definition:

pystac/pystac/extensions/datacube.py

Lines 143 to 150 in 6e8eb22

	class SpatialDimension(Dimension):
	@property
	def extent(self) -> list[float]:
	"""Extent (lower and upper bounds) of the dimension as two-dimensional array.
	Open intervals with ``None`` are not allowed."""
	return get_required(
	self.properties.get(DIM_EXTENT_PROP), "cube:dimension", DIM_EXTENT_PROP
	)

It is not possible to omit extent.

As a workaround, one has to set a extent=[None, None] definition, which does not provide much information, and looks more like an error than anything else for users. It would be more natural to omit the field entirely, especially when no specific range is known in advance.

[jsignell]

Good point! I just opened #1596 so fix this behavior.