PolytopeStructure - Structure for polytope.

   USAGE:

   pstruct = PolytopeStructure(matrix, rhs)
   pstruct = PolytopeStructure(matrix, rhs, vertices)
   pstruct = PolytopeStructure(matrix, rhs, vertices, faces)

   INPUT:

   matrix   is a m x d real array:
            the matrix of the linear inequalities defining
            the polytope (A, where Ax <= b)
   rhs      is a m x 1 real array:
            the right hand side of the linear inequalities defining
            the polytope (b, where Ax <= b)
   vertices is a d x n real array:  (optional)
            the list of n vertices in R^d
   faces    is a 1 x f cell array:  (optional)
            each cell contains a list of vertex numbers
            for a given face (3D polytopes only)

   OUTPUT:

   pstruct is a Polytopestructure,
           it consists of two primary fields, .matrix and
           .rhs; optionally, it can have two more fields,
           .vertices and .faces; the optional fields
           are not computed here, and they are primarily
           used by the graphics functions