An email conversation with Chuck Fresno turned up several forms of

An email conversation with Chuck Fresno turned up several forms of
symmetry which were not implemented in Solo. Now they are.

In the process I've completely retired symmetry_limit() on the
grounds that some of the new symmetries do not have a rectangular
base region; instead I determine the base region by going through
the grid and finding every square which is not transformed into a
lexicographically lower square by any symmetry operation. This means
that adding new symmetries is now _only_ a matter of encoding the
actual transformation rules.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5965 cda61777-01e9-0310-a592-d414129be87e