move en passant logic in a specific package
rename PositionFactory in PositionJsonifier
rename PositionJsonifier.createByData in importFromJson
add method PositionJsonifier.exportToJson
create PieceJsonifier to manage the build of a Piece from a json
add PieceJsonifier.exportToJson
add Board.exportPieces
add Game.exportToJson
correct a bug of inheritance

isPawnDoubleSquareDisplacement was not able to recognize that a pawn is an instance of Pawn.
that because when we init the pieces of the Board, we use PieceJsonifier.
That class instanciate a new Piecefactory for every pieces to create.
I do not know realy why but the inistiate Piece was not correct,
so en passant capture eligibility was not correct.

I can not reproduce the case in the test...
generate new dist