Dominosa: introduce a difficulty system.

Dominosa: introduce a difficulty system.

Currently, there are just two difficulty levels. 'Basic' is the same
as the old solver; 'Trivial' is the subset that guarantees the puzzle
can be solved using only the two simplest deductions of all: 'this
domino can only go in one place' and 'only one domino orientation can
fit in this square'.

The solver has also acquired a -g option, to grade the difficulty of
an input puzzle using this system.