Introduce halfspace_matrix_pair(ZZ, ...)

[alexej-jordan]

With halfspace_matrix_pair(ZZ, ...) we can ask for integral representations of its content. This solves #4682 .

In the following example we compare the default case to the integral case and the semi-integral case (i.e. b is not required to be integral, only A):

julia> p = polyhedron([1//3 1; 1 -2; -1 -1//5], [1//2, 1//2, 0])
Polyhedron in ambient dimension 2

julia> f = facets(p)
3-element SubObjectIterator{AffineHalfspace{QQFieldElem}} over the halfspaces of R^2 described by:
1//3*x_1 + x_2 <= 1//2
x_1 - 2*x_2 <= 1//2
-x_1 - 1//5*x_2 <= 0


julia> halfspace_matrix_pair(f)
(A = [1//3 1; 1 -2; -1 -1//5], b = QQFieldElem[1//2, 1//2, 0])

julia> halfspace_matrix_pair(ZZ, f)
(A = [2 6; 2 -4; -5 -1], b = ZZRingElem[3, 1, 0])

julia> halfspace_matrix_pair(ZZ, f; integral_bias = false)
(A = [1 3; 1 -2; -5 -1], b = QQFieldElem[3//2, 1//2, 0])

@lkastner can you play around and see if this meets your expactations? Also, from what I have tested this seemed to work properly for cones. Still, I did not like having to call halfspace_matrix_pair which gives us the redundant information that b is a zero vector. Do you think we should define a method of matrix for this context? I guess this should not conflict with any other definition of matrix, but if it does we could still find some appropriate function name.