Use a simpler, more efficient method to find the maximal solutions
Fix bug in partial_lt (smaller solutions should be considered greater)
solve2(return_all=True) should only return the maximal solutions
Add a test for something that doesn't work yet
min_sat returns all solutions, not just minimal ones

The minimality is handled in partial_lt. I haven't updated the docstring
yet. The upper bound will need to be raised.
Fix some tests