educe.stac: split and tidy stac.__init__

In light of better understanding of the Python module system.
Did some pylint and pep8 tidying while I was at it.

Note that there may be some API breakage from renaming constants.
Clean up RST code some more (API-breaking)

Note that RST tree node.type is now called node.nuclearity.
You should use is_nucleus() and is_satellite() instead
Tidy up RST test code and add binarize tests
Also update rst_dt.sdrt for API change
Update doc tree for educe.stac split up
Fix missing recursive step in RST binarisation
Hide rst_dt.transform.binarize
RST to dependency tree conversions
Some trivial RST/DT tests
Possibly missing rst test