Powerful Sudoku Solver

SUDOKU_LVL2 - A Sudoku Solver.

Usage : Sol=sudoku_lvl2(A)
Where A is a incomplete sudoku grid (9 x 9) represented as a 9 x 9 matrix of integers (0-9) with the empty cells being filled with zeros.

The algorithm presented here uses the following techniques.
(*) Filling a cell by candidate elimination
(*) Filling a cell by position exclusion for a given candidate
(*) Refining choices of candidates in a given cell by eliminating members
of locked pairs, locked triplets and so on..
(*) Filling the remaining cells by brute force method.

First 3 algorithms are of my own logic. The brute force algorithm is a big improvement over '8466'.

Note : Please note that the aim of this algorithm is to have the most comprehensive logic while trying to have a decent speed. Speed, though, is not the primary criterion.