Tests and documentation for Datacube extension

[lossyrob]

As part of #309 the Datacube extension was implemented (closing #30). There's testing and documentation left to do on this extension that should go in before the 1.0 release.

[duckontheweb]

Moving this out to 1.3.0 release since I won't have time to work on it for 1.2.0

[duckontheweb]

Moving this to the next release.

[guidorice]

I am in process of adding more unit tests to pystac/extensions/datacube.py, but it occurs to me that Dimension takes a dict in it's constructor, so also should have create and apply, same as in #576 .

I think it might turn into a bit of a refactor of some of the datacube classes, because from_dict here seems awkward being defined in the Abstract Base Class.

Instead, from_dict, create and apply could be implemented by the concrete subclasses. Let me know if any opinions pro or con- I am not experienced using the ABC package.

cc @duckontheweb