* Fixed the groups.py script so it really does what is described (the
old code had the same result for groups, group_lines and
group_lists).
* Updated the corresponding example in the doc to match.
* Updated usage so it only references importing the final list of
lists produced. Trying to use some of the mid-points can have
unpredictable results (this is part of the problem with work
arounds).
* Added groups selection work around code.
* Intended for use as a module to be imported by other scripts,
usually with "from groups import group_lists" or "from groups import
group_lines" or similar.
