decomposer.cc: avoid use of half-open cones

decomposer.cc: avoid use of half-open cones

Since we are only dealing with half-open simplicial cones
and since generating functions of non-pointed cones are zero,
we can replace a cone pos { u_1,...,u_{i-1},u_i,u_{i+1}...,u_d },
where the facet opposite u_d is open by
    pos { u_1,...,u_{i-1},u_{i+1}...,u_d }
	-pos { u_1,...,u_{i-1},-u_i,u_{i+1}...,u_d }
where the first cone is not pointed (and can therefore be ignored)
and the facet opposite -u_i in the second cone is closed.