Added type Difficulty.  Will try to rate puzzles on it.  Starting to
histogram different puzzles.

Next step, pull out show array to a type and make different show classes
Changed the way masks are generated.  This allows for a more interesting
selection of masks, rather than quite so random.  Adding in difficulty
histogramming, to see what combinations really make sense.
Adding more mask types.

Updated the difficultyHisto test to test them all.  Also changed
Difficulty so that equal is determined only by D and G.
Additional histogramming on tries.