Add a new deduction to Easy level, which is as small as I can make it

to have the effect of enabling large Easy-level grids to be
constructed in all grid types. Without this, some generations at Easy
level (e.g. 'loopy --generate 1 7x7t9de') can spin forever because
_even with all clues filled in_ the generated grids can't be solved at
that level.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9143 cda61777-01e9-0310-a592-d414129be87e