Implemented a couple more reasoning modes for Extreme difficulty

Implemented a couple more reasoning modes for Extreme difficulty
level: positional set elimination (which is so obvious I really
should have thought of it myself, though it's tricky to spot) and
forcing chains (which are a type of one-level proof by
contradiction, findable through a simple breadth-first search
without requiring recursion, but so ludicrously powerful that they
are able to solve _two thirds_ of grids that the pre-Extreme Solo
generated and rated as Unreasonable).

Of course this makes Unreasonable mode harder still...

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6239 cda61777-01e9-0310-a592-d414129be87e