removed caching of slices
need only a List of STLObjects to slice
moved shield stl creation to Purge
refactored hatch()
split math operations from BooleanGrid into new class

BooleanGridMath
refactored constructors
refactorings Integer2DRectangle, DigitalDifferentialAnalyzer, Point2D

also added missing file headers
extracted painting a csg expression from BooleanGrid to its own class
extraced hatching from BooleanGrid to a new class Hatcher
extracted flood filling from BooleanGrid to its own class FloodFiller
BooleanGridList is now immutable and simplified support calculation