Correct slicing of HorizontalCylinder, and improve numerics

In auto_slicing.py test, we currently have

        self.runPlainFF(ba.HorizontalCylinder(5., 19.)) # TODO restore tol=1e-13
        #self.runPlainFF(ba.HorizontalCylinder(5., 19., -4., 4.)) TODO wrong!
        #self.runPlainFF(ba.HorizontalCylinder(6., 19., -3., 3.))

TODO:

• correct slicing for truncated horizontal cylinders
• improve numerics of ff computation, and restore tol=1e-13 for all tests