Removed column and row permutations as discussed in 19/01/2013 by adding
col_freq and row_freq tables to the model
Solves the 16x16 grid. 20/01/2013. Restricted the values of the col and row freq tables to be more accurate (middle +- 2 rather than 1..size). Reordered the constraints, as it makes a difference to the number of backtracks etc.
Added search annotations to search amongst the most likely row and col freqs first