Adding Variants

[SudokuMonster]

Added several variants
visibleCellIndex and visibleCellSets are updated by calling changeVisibleCells. isVanilla() will be used throughout the code to minimizes un-necessary check for different variants when in vanilla sudoku mode.
Disjoint groups is currently has each block position in a different colour. I'm hoping that I can use a B&W texturedPaint instead in the future.

Decided not to allow simultaneous "Extra groups" as I'm out of colours! All can be used from command line though

[SudokuMonster]

Region extended for each variant constraint
Changes to techniques to accommodate variants
isVanilla and isVLatin are used to minimize checks for each individual variant
Several new options to control variants from command line

-D --isDG
-E --isCD
-K --isAsterisk
-R --isGirandola
-W --isWindows
-X --isX

[SudokuMonster]

-n --isAntiKnight
-k --isAntiKing
-O --isToroidal
-c --isNC

c1 for NC and c2 for NC+
All default to 0 (disabled)

New techniques (and therefore 2 more characters in --techs)

forcingCellNC
lockedNC

These are enabled by default in all settings but will only be used if whichNC() > 0